[Update: May. 09. 2023]
Compiling test of MEGAsync-4.9.4.0_Win on slackware64-current(15.0+) with FFMPEG-5.1.3

I updated the package against current ffmpeg-5.x environment.
Package was compiled successfully on slackware64-15.0+ with ffmpeg-5.1.3 and is working fine so far.


MEGAsync.SlackBuild
[code]
#!/bin/sh

# Slackware build script for MEGASync input plugin

# Copyright 2017,2018 Felipe Bugno <capent@yahoo.com>
#
# All rights reserved.
#
# Redistribution and use of this script, with or without modification, is
# permitted provided that the following conditions are met:
#
# 1. Redistributions of this script must retain the above copyright
#    notice, this list of conditions and the following disclaimer.
#
#  THIS SOFTWARE IS PROVIDED BY THE AUTHOR "AS IS" AND ANY EXPRESS OR IMPLIED
#  WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF
#  MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.  IN NO
#  EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
#  SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
#  PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS;
#  OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY,
#  WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR
#  OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF
#  ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
#
# 2023-01-26 : 4.8.6.0 (1jw) -- MEGAsync-4.8.6.0_Win source with sdk-4.9.0b
# 2023-01-31 : 4.8.6.0 (2jw) -- Re-compile against libRaw-0.21.1 (libraw.so.23)
# 2023-02-04 : 4.8.7.0 (1jw) -- MEGAsync-4.8.7.0_Win source with sdk-4.9.0c
# 2023-02-28 : 4.8.8.0 (1jw) -- MEGAsync-4.8.8.0_Win source with sdk-4.14.0
# 2023-03-21 : 4.9.1.0 (1jw) -- MEGAsync-4.9.1.0_Win source with sdk-4.16.0c
# 2023-04-29 : 4.9.3.0 (1jw) -- MEGAsync-4.9.3.0_Win source with sdk-4.17.1
# 2023-05-09 : 4.9.4.0 (1jw) -- MEGAsync-4.9.4.0_Win source with sdk-4.17.1d

PRGNAM=MEGAsync
VERSION=${VERSION:-4.9.4.0}
SDK=${SDK:-4.17.1d}
FFMPEG5_PATCHREV=${FFMPEG5_PATCHREV:-4.9.0.0}
BUILD=${BUILD:-1}
TAG=${TAG:-jw}

if [ -z "$ARCH" ]; then
  case "$( uname -m )" in
    i?86) ARCH=i586 ;;
    arm*) ARCH=arm ;;
       *) ARCH=$( uname -m ) ;;
  esac
fi

CWD=$(pwd)
TMP=${TMP:-/tmp/SBo}
PKG=$TMP/package-$PRGNAM
OUTPUT=${OUTPUT:-/tmp}

if [ "$ARCH" = "i586" ]; then
  SLKCFLAGS="-O2 -march=i586 -mtune=i686"
  LIBDIRSUFFIX=""
elif [ "$ARCH" = "i686" ]; then
  SLKCFLAGS="-O2 -march=i686 -mtune=i686"
  LIBDIRSUFFIX=""
elif [ "$ARCH" = "x86_64" ]; then
  SLKCFLAGS="-O2 -fPIC"
  LIBDIRSUFFIX="64"
else
  SLKCFLAGS="-O2"
  LIBDIRSUFFIX=""
fi

set -e

rm -rf $PKG
mkdir -p $TMP $PKG $OUTPUT
cd $TMP
rm -rf $PRGNAM-$VERSION\_Win
tar xvf $CWD/$PRGNAM-$VERSION\_Win.tar.gz
cd $PRGNAM-$VERSION\_Win/src/MEGASync/mega
tar --strip-components=1 -xvf $CWD/sdk-$SDK.tar.gz
cd ../../../
chown -R root:root .
find -L . \
 \( -perm 777 -o -perm 775 -o -perm 750 -o -perm 711 -o -perm 555 \
  -o -perm 511 \) -exec chmod 755 {} \; -o \
 \( -perm 666 -o -perm 664 -o -perm 640 -o -perm 600 -o -perm 444 \
  -o -perm 440 -o -perm 400 \) -exec chmod 644 {} \;

cd src/MEGASync/mega
patch -p1 < $CWD/megasync-4.5.0.0_pdfium.patch

./autogen.sh
CFLAGS="$SLKCFLAGS" \
CXXFLAGS="$SLKCFLAGS" \
./configure \
  --prefix=/usr \
  --datarootdir=/usr \
  --libdir=/usr/lib${LIBDIRSUFFIX} \
  --sysconfdir=/etc \
  --localstatedir=/var \
  --mandir=/usr/man \
  --docdir=/usr/doc/$PRGNAM-$VERSION \
  --build=$ARCH-slackware-linux \
  --with-cares \
  --with-cryptopp \
  --with-curl \
  --with-sodium \
  --with-sqlite \
  --with-zlib \
  --with-termcap \
  --with-freeimage \
  --enable-chat \
  --enable-gcc-hardening

# Apply ffmpeg5 fix patch
cd ../../../
patch -p1 < $CWD/megasync-${FFMPEG5_PATCHREV}_ffmpeg5.patch

cd src
#cd ../../../src
qmake CONFIG+="release" QMAKE_CXXFLAGS="$SLKCFLAGS" MEGA.pro
lrelease MEGASync/MEGASync.pro
make
install -m 755 -p -D $TMP/$PRGNAM-$VERSION\_Win/src/MEGASync/megasync $PKG/usr/bin/megasync
install -m 644 -p -D $TMP/$PRGNAM-$VERSION\_Win/src/MEGASync/platform/linux/data/megasync.desktop $PKG/usr/share/applications/megasync.desktop
install -m 644 -p -D $TMP/$PRGNAM-$VERSION\_Win/src/MEGASync/platform/linux/data/icons/hicolor/16x16/apps/mega.png $PKG/usr/share/icons/hicolor/16x16/apps/mega.png
install -m 644 -p -D $TMP/$PRGNAM-$VERSION\_Win/src/MEGASync/platform/linux/data/icons/hicolor/32x32/apps/mega.png $PKG/usr/share/icons/hicolor/32x32/apps/mega.png
install -m 644 -p -D $TMP/$PRGNAM-$VERSION\_Win/src/MEGASync/platform/linux/data/icons/hicolor/48x48/apps/mega.png $PKG/usr/share/icons/hicolor/48x48/apps/mega.png
install -m 644 -p -D $TMP/$PRGNAM-$VERSION\_Win/src/MEGASync/platform/linux/data/icons/hicolor/128x128/apps/mega.png $PKG/usr/share/icons/hicolor/128x128/apps/mega.png
install -m 644 -p -D $TMP/$PRGNAM-$VERSION\_Win/src/MEGASync/platform/linux/data/icons/hicolor/256x256/apps/mega.png $PKG/usr/share/icons/hicolor/256x256/apps/mega.png

find $PKG -print0 | xargs -0 file | grep -e "executable" -e "shared object" | grep ELF \
  | cut -f 1 -d : | xargs strip --strip-unneeded 2> /dev/null || true

mkdir -p $PKG/usr/doc/$PRGNAM-$VERSION
cd $TMP/$PRGNAM-$VERSION\_Win
cp -a  README.md LICENCE.md CREDITS.md $PKG/usr/doc/$PRGNAM-$VERSION
cp $CWD/megasync-4.5.0.0_pdfium.patch $CWD/megasync-${FFMPEG5_PATCHREV}_ffmpeg5.patch $PKG/usr/doc/$PRGNAM-$VERSION
cat $CWD/$PRGNAM.SlackBuild > $PKG/usr/doc/$PRGNAM-$VERSION/$PRGNAM.SlackBuild

mkdir -p $PKG/install
cat $CWD/slack-desc > $PKG/install/slack-desc
cat $CWD/doinst.sh > $PKG/install/doinst.sh

cd $PKG
/sbin/makepkg -l y -c n $OUTPUT/$PRGNAM-$VERSION-$ARCH-$BUILD$TAG.${PKGTYPE:-txz}

MEGAsync.info
[code]
PRGNAM="MEGAsync"
VERSION="4.9.4.0"
HOMEPAGE="https://github.com/meganz/MEGAsync/"
DOWNLOAD="https://github.com/meganz/MEGAsync/archive/v4.9.4.0_Win/MEGAsync-4.9.4.0_Win.tar.gz \
 https://github.com/meganz/sdk/archive/v4.17.1d/sdk-4.17.1d.tar.gz"
MD5SUM="77e8fac92b50fc72044c58a7826a3b36 \
 09e1926a06ab62ddef9b2dd3846c6ba7"
DOWNLOAD_x86_64=""
MD5SUM_x86_64=""
REQUIRES="FreeImage libmediainfo"
MAINTAINER="Felipe Bugno"
EMAIL="capent@yahoo.com"

megasync-4.5.0.0_pdfium.patch
[code]
diff --git a/bindings/qt/sdk.pri b/bindings/qt/sdk.pri
index 83ddc2e2f..46aa06d33 100644
--- a/bindings/qt/sdk.pri
+++ b/bindings/qt/sdk.pri
@@ -255,6 +255,11 @@ CONFIG(USE_PDFIUM) {
                 DEFINES += HAVE_PDFIUM
                 LIBS += -lpdfium
             }
+            else:exists(/usr/include/pdfium/fpdfview.h) {
+                DEFINES += HAVE_PDFIUM
+                INCLUDEPATH += /usr/include/pdfium
+                LIBS += -lpdfium
+            }
         }
         else {#win/mac
             DEFINES += HAVE_PDFIUM

megasync-4.9.0.0_ffmpeg5.patch
[code]
--- MEGAsync-4.9.0.0_Linux/src/MEGASync/mega/src/gfx/freeimage.cpp    2023-03-04 07:13:58.000000000 +0900
+++ MEGAsync-4.9.0.0_Linux.new/src/MEGASync/mega/src/gfx/freeimage.cpp    2023-03-06 23:06:33.669459409 +0900
@@ -313,7 +313,7 @@
 
     // Find decoder for video stream
     AVCodecID codecId = codecParm->codec_id;
-    AVCodec* decoder = avcodec_find_decoder(codecId);
+    auto decoder = avcodec_find_decoder(codecId);
     if (!decoder)
     {
         LOG_warn << "Codec not found: " << codecId;
@@ -330,7 +330,7 @@
 
     // Force seeking to key frames
     formatContext->seek2any = false;
-    videoStream->skip_to_keyframe = true;
+    //videoStream->skip_to_keyframe = true;
     if (decoder->capabilities & CAP_TRUNCATED)
     {
         codecContext->flags |= CAP_TRUNCATED;

slack-desc
[code]
# HOW TO EDIT THIS FILE:
# The "handy ruler" below makes it easier to edit a package description.
# Line up the first '|' above the ':' following the base package name, and
# the '|' on the right side marks the last column you can put a character in.
# You must make exactly 11 lines for the formatting to be correct.  It's also
# customary to leave one space after the ':' except on otherwise blank lines.

        |-----handy-ruler------------------------------------------------------|
MEGAsync: MEGAsync (mega cloud storage service)
MEGAsync:
MEGAsync: MEGAsync open source release for MEGA cloud storage services
MEGAsync:
MEGAsync:
MEGAsync: Compiled using default Slackware Qt4.
MEGAsync:
MEGAsync: Homepage: https://github.com/meganz/MEGAsync
MEGAsync: Service: https://mega.nz/
MEGAsync:
MEGAsync:

do-inst.sh
[code]
if [ -x /usr/bin/update-desktop-database ]; then
  /usr/bin/update-desktop-database -q usr/share/applications >/dev/null 2>&1
fi

if [ -e usr/share/icons/hicolor/icon-theme.cache ]; then
  if [ -x /usr/bin/gtk-update-icon-cache ]; then
    /usr/bin/gtk-update-icon-cache -f usr/share/icons/hicolor >/dev/null 2>&1
  fi
fi

---
[Update] Nov.18.2022  : TEST build against ffmpeg-5.1.2 with new patch(megasync-4.7.3.0_ffmpeg5.patch)
[Update] Jan.26.2023 : TEST build against ffmpeg-5.1.2 with MEGAsync-4.8.6.0_Win source
[Update] Feb.04.2023 : TEST build against ffmpeg-5.1.2 with MEGAsync-4.8.7.0_Win source
[Update] Feb.28.2023 : TEST build against ffmpeg-5.1.2 with MEGAsync-4.8.8.0_Win source
[Update] Mar.22.2023 : TEST build against ffmpeg-5.1.2 with MEGAsync-4.9.1.0_Win source
[Update] May.09.2023 : TEST build against ffmpeg-5.1.3 with MEGAsync-4.9.4.0_Win source