mesa: fix make tarballs

mesa: fix make tarballs

Current method of generating distribution tar-balls involves manually
invoking make + target name in the appropriate places. This temporary
solution is used until we get 'make dist' working.

Currently it does not work, as in order to have the target (which is
also a filename) available in the final Makefile we need to add a PHONY
target + use the correct target name.

Cc: "10.2 10.3" <mesa-stable@lists.freedesktop.org>
Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>