I've confirmed that cups upstream issue#998 was solved with Fork source Fix updating cgiSetVariable variables (issue OpenPrinting#998) .
Thanks to Gerben100/cups.

Here is simple steps of creating the cups package for slackware64-current.
 
Step-1. Get slackware's official slackbuild
 # mkdir <MY_WORKING_DIR>
 # cd <MY_WORKING_DIR>
 # wget -nH --cut-dirs=4 -r --no-parent https://mirror.slackware.jp/slackware/slackware64-current/source/ap/cups/
 
Step-2. Get the Gerben100's git source and create it's tar ball for packaging
  fetch-cups-2.4.10-git_commit-source.sh
[code]
#!/bin/bash
BASE_VER="2.4.10"
# Read commit version number
echo -n COMMIT_NBR:
read COMMIT

# Set the first "7"number of the Commit-number as COMMIT_SHORT_VER
COMMIT_SHORT_VER=${COMMIT:0:7}

# fetch the commit git source (From Gerben100's Fork)
wget https://github.com/Gerben100/cups/archive/${COMMIT}/cups-${COMMIT}.tar.gz

# Rename original git source name to "cups-${BASE_VER}.${COMMIT_SHORT_VER}"
tar xvf cups-${COMMIT}.tar.gz && mv cups-${COMMIT} cups-${BASE_VER}.$COMMIT_SHORT_VER
# Create tar.xz file for my packaging work
tar cJvf cups-${BASE_VER}.${COMMIT_SHORT_VER}.tar.xz cups-${BASE_VER}.${COMMIT_SHORT_VER}/ && rm -rf cups-${BASE_VER}.${COMMIT_SHORT_VER}/ \
&& rm -rf cups-${COMMIT}.tar.gz

echo "...DONE!"

 Copy this fetch-cups-2.4.10-git_commit-source.sh script into <MY_WORKING_DIR>/cups
 # cd <MY_WORKING_DIR>/cups
 # rm cups-2.4.10.tar.lz
 # chmod +x fetch-cups-2.4.10-git_commit-source.sh
 # sh fetch-cups-2.4.10-git_commit-source.sh
 Then, ENTER the commit number: 7ac2f98dc3e9c049e8597f1988f2b64946952230

Step-3. Create package
 Just run SlackBuild
 # sh cups.SlackBuild

 This will create a new cups package "cups-2.4.10.7ac2f98-x86_64-1.txz".
 If you want to use your own TAG, please change "BUILD=${BUILD:-1}" to your own one.

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