[Piglit] libepoxy conversion v2

[Chad Versace]

Eric said:
> > But then there's the broken configure caching that makes it so you have
> > to git clean regularly, and the broken library directory handling.  I
> > mean, look at the workarounds in chad's "[Piglit] [ANNOUNCE] waffle >=
> > 1.3 required, you may need to rerun CMake".  This build system is
> > totally broken.

I discovered how to uncache CMake's pkg-config queries and force CMake
to rerun pkg-config anytime the CMake files change.

I don't know how trustworthy my approach is yet, so I'm letting it bake
in a personal project until I feel it's safe.