configure: use AC_GNU_SOURCE
configure: use AC_GNU_SOURCE

This adds the AC_GNU_SOURCE macro to configure.ac to ensure _GNU_SOURCE
is defined while compiling Cogl. This is required to use memmem for
example.

Reviewed-by: Neil Roberts <822ec91a7ef82248fd36162ca8ac3283eba70848@linux.intel.com>

djdeath/cogl,gcampax/cogl,gcampax/cogl,gcampax/cogl,djdeath/cogl-android,djdeath/cogl,djdeath/cogl,djdeath/cogl-android,gcampax/cogl,djdeath/cogl-android