mesa: Allow custom text to be inserted in version string at buildtime

This allows maintainers/packagers/testers to tag the build with
information that will be reported by GL_VERSION.

If the environemt variable or make variable MESA_VERSION_STRING_EXTRA is
set, then its values will appear in the GL_VERSION string immediately
after "Mesa X.Y" and before "(git-xxxxxxx)".

This patch implements supports MESA_VERSION_STRING_EXTRA only for Android.
Other build systems are left as an excercise.

Change-Id: I06432f6bd66546a3a62abf22347a2a0fb381ab74
CC: Matt Turner <mattst88@gmail.com>
CC: Tapani Pälli <tapani.palli@intel.com>
Signed-off-by: Chad Versace <chad.versace@linux.intel.com>