mesa: Insert custom text into version string at buildtime

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

If during build time (not configure time), the environemt variable or make
variable MESA_VERSION_STRING_EXTRA is set, then its value will appear in
the GL_VERSION string immediately after "Mesa X.Y" and before
"(git-xxxxxxx)".

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

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