lavc/pthread: do not re-define _GNU_SOURCE if already defined.

This fixes the following warning with GCC:
    libavcodec/pthread.c:35:0: warning: _GNU_SOURCE redefined [enabled by default]
    <command-line>::0: note: this is the location of the previous definition

The reason of the presence of this flag is:
    % pkg-config --cflags sdl
    -D_GNU_SOURCE=1 -D_REENTRANT -I/usr/include/SDL

libavcodec/pthread.c

@@ -32,7 +32,9 @@
 #include "config.h"
 
 #if HAVE_SCHED_GETAFFINITY
-#define _GNU_SOURCE
+#ifndef _GNU_SOURCE
+# define _GNU_SOURCE
+#endif
 #include <sched.h>
 #endif
 #if HAVE_GETPROCESSAFFINITYMASK