[cogl] configure: use AC_GNU_SOURCE

[Robert Bragg <rbragg src gnome org>]

commit 850c8305a26cb370a72884ac8a645f71d506b239
Author: Robert Bragg <robert linux intel com>
Date:   Wed Aug 15 13:19:22 2012 +0100

    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 <neil linux intel com>

 configure.ac |    1 +
 1 files changed, 1 insertions(+), 0 deletions(-)
---
diff --git a/configure.ac b/configure.ac
index 6764ee7..d534cf7 100644
--- a/configure.ac
+++ b/configure.ac
@@ -71,6 +71,7 @@ AC_CONFIG_SRCDIR(cogl/cogl.h)
 AC_CONFIG_AUX_DIR([build])
 AC_CONFIG_MACRO_DIR([build/autotools])
 AC_CONFIG_HEADERS(config.h)
+AC_GNU_SOURCE
 
 dnl ================================================================
 dnl Required versions for dependencies