Recompiling ffmpeg best practices?

I've been trying to run AlienBOB's ffmpeg6 script on Slackware 15 Stable, and I've been running into some problems. The code below is the STDERR output of the script, with it running well until it tries to apply the Vulkan diff. A user informed me that Slackware 15, if up-to-date, should include Vulkan, but it didn't show up when I ran slackpkg update and install-new, so I'm not sure if I'm missing something. Searching Slackbuilds.org returned nothing for Vulkan on Slackware 15, which implies it's included in 15...

Code:Hunk #1 succeeded at 179 (offset 49 lines).
Hmm... Ignoring the trailing garbage.
done
Hmm... Looks like a unified diff to me...
The text leading up to this was:
--------------------------
|--- ./configure.orig 2023-03-26 09:21:00.000000000 -0500
|+++ ./configure 2023-08-29 13:13:08.176019532 -0500
--------------------------
patching file configure
Using Plan A...
Hunk #1 succeeded at 6705 with fuzz 2 (offset 154 lines).
done
Hmm... Looks like a unified diff to me...
The text leading up to this was:
--------------------------
|diff '--color=auto' -rupN ffmpeg.orig/libavformat/avformat.h ffmpeg/libavformat/avformat.h
|--- ffmpeg.orig/libavformat/avformat.h 2022-08-19 17:42:47.323422603 +0200
|+++ ffmpeg/libavformat/avformat.h 2022-08-19 17:42:51.347130436 +0200
--------------------------
patching file libavformat/avformat.h
Using Plan A...
Hunk #1 succeeded at 1030 with fuzz 2 (offset -98 lines).
Hmm... The next patch looks like a unified diff to me...
The text leading up to this was:
--------------------------
|diff '--color=auto' -rupN ffmpeg.orig/libavformat/mux_utils.c ffmpeg/libavformat/mux_utils.c
|--- ffmpeg.orig/libavformat/mux_utils.c 2022-08-19 17:42:47.346758108 +0200
|+++ ffmpeg/libavformat/mux_utils.c 2022-08-19 17:47:28.549589002 +0200
--------------------------
patching file libavformat/mux_utils.c
Using Plan A...
Hunk #1 succeeded at 40 with fuzz 2 (offset 3 lines).
done
ERROR: vulkan requested but not found

If you think configure made a mistake, make sure you are using the latest
version from Git. If the latest version fails, report the problem to the
ffmpeg-user@ffmpeg.org mailing list or IRC #ffmpeg on irc.libera.chat.
Include the log file "ffbuild/config.log" produced by configure as this will help
solve the problem.
Makefile:186: /tests/Makefile: No such file or directory
make: *** No rule to make target '/tests/Makefile'. Stop.
Makefile:186: /tests/Makefile: No such file or directory
make: *** No rule to make target '/tests/Makefile'. Stop.
** Adding ffmpeg presets:
** Fixing up pkgconfig files:
ls: cannot access '/tmp/build/package-ffmpeg/usr/lib/pkgconfig/*.pc': No such file or directory
cp: cannot stat 'doc/*.html': No such file or directory
./ffmpeg.SlackBuild FAILED at line 2029!The main thesis of what I want to ask is; with regards to recompiling large programs like ffmpeg, which place their fingers in many pies - depending upon myriad dynamic libraries, of whose versions change according to whether you're using ffmpeg4.x, ffmpeg5.x or ffmpeg6.x - I'm wondering what is the general best practice for recompiling such programs on Slackware?

Has anyone successfully got AlienBob's ffmpeg6.x SlackBuild working on 15 Stable? If this is infeasible and would simply introduce too many breakages, I'm wondering if it would be better to extend his ffmpeg4.x script to include the likes of updated libdav1d and AV1 encoder libraries? Or might it be better to edit his ffmpeg5.x or ffmpeg6.x scripts to work with already-installed library versions on 15 Stable?