Add pkgconfig data (v2)
Created by: kbrenneman
This is based on the patch in #86.
This adds pkg-config files for all of the application-facing libraries in libglvnd, including the libglvnd-specific libGLX and libOpenGL.
Unlike the files from Mesa, the version number in each file is the API version, not the package version. That'll require changes to any other builds that specify a particular Mesa version. But, we can't reasonably track Mesa's version number in libglvnd, so that would break anyway.
After that, libglvnd will also now install the GL, GLES, GLX, and EGL header files. With that, libglvnd alone should be all you need to build an OpenGL application.