build: Only allow GLX when finding the x11 pc file
build: Only allow GLX when finding the x11 pc file

You can refine earlier the ALLOW_GLX variable when enabling OpenGL builds:
No x11.pc? no GLX!

djdeath/cogl-android,collects/cogl,gcampax/cogl,gcampax/cogl,djdeath/cogl-android,djdeath/cogl-android,Distrotech/cogl,Distrotech/cogl,spatulasnout/cogl,spatulasnout/cogl,gcampax/cogl,djdeath/cogl,djdeath/cogl,gcampax/cogl,Distrotech/cogl,djdeath/cogl,collects/cogl,Distrotech/cogl