Add more package metadata to the pkg-config files

Add more package metadata to the pkg-config files

The pkg-config files have been filled in more thoroughly to allow users
to use mesa more effectively. By adding metadata to Requires.private,
Libs.private and Cflags, we can ensure that all the libraries and
headers will be found in all situations. However, the full substitutions
are only done when using the configure script.

This also fixes the glu pkg-config file to account for using GL or
OSMesa.

Fixes bug 18161.