[rglfw] fix: supress warning: "_GNU_SOURCE" redefined

When compiling rglfw.c with GCC or Clang following warning appears:

In file included from rglfw.c:99:
external/glfw/src/posix_poll.c:27:9: warning: "_GNU_SOURCE" redefined
   27 | #define _GNU_SOURCE
      |         ^~~~~~~~~~~

This commit suppresses said warning by checking,
if _GNU_SOURCE has NOT been defined, and only then defining it.

Since said macro has been always defined in posix_poll and wl_window,
this commit does not change any behavior at all (it just suppresses the
warning, that's it).

src/external/glfw/src/posix_poll.c

@@ -24,7 +24,9 @@
 //
 //========================================================================
 
-#define _GNU_SOURCE
+#if !defined(_GNU_SOURCE)
+    #define _GNU_SOURCE
+#endif
 
 #include "internal.h"
 

src/external/glfw/src/wl_window.c

@@ -25,7 +25,9 @@
 //
 //========================================================================
 
-#define _GNU_SOURCE
+#if !defined(_GNU_SOURCE)
+    #define _GNU_SOURCE
+#endif
 
 #include "internal.h"