Log Message:
Compute version of built-in Mesa without using "glxinfo", which is broken
when the capabilities of the running X11 server differ from the X11 server
on the system for which the package is meant.  We now try to guess the
Mesa version number based on which OpenGL specification is implemented.

Separate out the logic into its own file, Mesa/version.mk, and use it in
MesaLib's and glu's buildlink2.mk files.

Note that Mesa/version.mk should only be used by the Mesa packages and is
_not_ for general use.