build: Require libepoxy

build: Require libepoxy

If we want to use OpenGL in GDK then we have two choices; either:

  - find the GL headers on each platform
  - do extension discovery
  - implement all the crazy dlopen()/dlsym() dispatch tables

*or* use libepoxy, which shields us from all this madness and provides a
decent layer for GL clients to use, without creating its own namespace.

Epoxy is also used by other projects, like Xorg and piglit, and it's
portable to all the platforms GDK cares about.

https://bugzilla.gnome.org/show_bug.cgi?id=119189