configure: Drop broken workarounds for missing pkg-config

Any reasonably-modern system should have versions of things like OpenGL
libraries installed with pkg-config libraries.

Regardless, the checks we had in place here for missing gl.pc files
were untested and obviously not very useful, (they didn't actually
look around anywhere for GL headers nor for GL libraries).

We're better off not even pretending to be able to find things without
pkg-config.