Merge pull request #66648 (improve xkbvalidate)

This allows xkbvalidate to be compiled via Clang and also has a few
other portability improvements, eg. it now can even be compiled on OS X,
even though it's probably not needed there.

In addition, I changed the binary name so that it matches the package
name.

I'm merging this in right now, because there is only the xserver NixOS
module where this is used, so the risk of a catastrophic breakage is
very low.

Checks and build done by ofborg also ran successfully and I also did a
few local tests (eg. running via valgrind to avoid leaks) to make sure
it's still working properly.