Bug #9555: Always define _GNU_SOURCE in glibc environments.

This keeps us from having to define _POSIX_C_SOURCE, _BSD_SOURCE, and
_XOPEN_SORUCE to get the C environment we want in different places.  It also
fixes the build on linux due to RTLD_DEFAULT having not been defined.

os/utils.c

@@ -53,23 +53,6 @@ OR PERFORMANCE OF THIS SOFTWARE.
 #include <dix-config.h>
 #endif
 
-#ifndef __GLIBC__
-#include <time.h>
-#else
-/* The world's most shocking hack, to ensure we get clock_gettime() and
- * CLOCK_MONOTONIC. */
-#ifdef _POSIX_C_SOURCE
-#define _SAVED_POSIX_C_SOURCE _POSIX_C_SOURCE
-#undef _POSIX_C_SOURCE
-#endif
-#define _POSIX_C_SOURCE 199309L
-#include <time.h>
-#undef _POSIX_C_SOURCE
-#ifdef _SAVED_POSIX_C_SOURCE
-#define _POSIX_C_SOURCE _SAVED_POSIX_C_SOURCE
-#endif
-#endif /* __linux__ */
-
 #ifdef __CYGWIN__
 #include <stdlib.h>
 #include <signal.h>
@@ -80,6 +63,7 @@ OR PERFORMANCE OF THIS SOFTWARE.
 #endif
 #include <X11/Xos.h>
 #include <stdio.h>
+#include <time.h>
 #include "misc.h"
 #include <X11/X.h>
 #define XSERV_t