Log Message:
firefox78: Honor user's compiler choice again, don't require Python 2.

The python 2 dependency was seemingly removed in Firefox 78.0 so we
can remove those old hacks.

Firefox needs clang for some unknown part of the build process (rust
related?), even if building with GCC.

The previous solution in pkgsrc was to force the use of clang, because
pkgsrc provides cwrappers which provided gcc-as-clang, which broke
everything. Instead, override the clang wrapper with the actual clang
executable.

This means the majority of the build happens with GCC (or ccache, distcc,
whatever the user chooses, rather than overriding it with clang). Should help
sparc64, where clang doesn't work too well.

Full build tested on NetBSD/amd64.