Add pkg-config files for EGL, GL, GLES, and GLX.

This preserves the names from Mesa's gl, glesv1_cm, glesv2, and egl pkgconfig
data, and for compatibility with that, gl.pc gives you libGL.

Also added glx.pc and opengl.pc for libglvnd's libGLX and libOpenGL libraries.
If newer applications want to explicitly depend on libGLX and the appropriate
rendering API they can ask for the new pkgconfig names.

Note that the version numbers in each file are the API versions that each
library exposes, not the libglvnd package version.