Add comment to Makefile-xorg-gtest.am about compiling with -w
Add comment to Makefile-xorg-gtest.am about compiling with -w

Users of xorg-gtest will not want warnings or failed builds due to
issues in gtest or xorg-gtest.

Note that the internal build of the xorg-gtest example does not use the
-w flag when building examples, so we will still see warnings when
xorg-gtest itself is built.

Signed-off-by: Chase Douglas <69e02ec421aa70a149511825d897f2549ceee9a2@canonical.com>
Reviewed-by: Peter Hutterer <b9c50af7afa7642f6f854434cda9ffe54ece8b85@who-t.net>

freedesktop-unofficial-mirror/xorg__test__xorg-gtest,freedesktop-unofficial-mirror/xorg__test__xorg-gtest,freedesktop-unofficial-mirror/xorg__test__xorg-gtest