gbm: Define _DEFAULT_SOURCE to avoid warning
gbm: Define _DEFAULT_SOURCE to avoid warning

glibc 2.19 introduced _DEFUAULT_SOURCE as a replacement for _BSD_SOURCE,
and deprecates _BSD_SOURCE with an annoying warning.  Defining both is
how you're supposed to transition so let's do that.  It gets rid of the
warning and we can figure out when/if we can drop _BSD_SOURCE later.

Signed-off-by: Kristian Høgsberg <ca9b5b1e27d4fa7f4ba6848bdd08ca52cfd6330c@bitplanet.net>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler