tvheadend.SlackBuild fails with x265 error

I'm trying to build tvheadend using the slackbuild and get errors with the default tvheadend-3.4patch1 tarball. They seem to be related to using gcc/g++ 10.X so I grabbed a copy of tvheadend-4.99 and it will compile until it fails while building ffmpeg with the error message: "ERROR: x265 not found using pkg-config". There's a few old discussions about this error but nothing that I can see a solution in.

Has anyone successfully compiled tvheadend on an up to date slackware-current system? If so, what's the secret? :)

Thanks,
Jim