Increase _POSIX_C_SOURCE to 200112L

CLOCK_MONOTONIC appeared in IEEE Std. 1003.1-200x, it was not part of
POSIX.1b (the 1993 version), and FreeBSD treats it accordingly.

backend/drm/drm.c

@@ -1,4 +1,4 @@
-#define _POSIX_C_SOURCE 199309L
+#define _POSIX_C_SOURCE 200112L
 #include <assert.h>
 #include <drm_mode.h>
 #include <EGL/egl.h>

backend/multi/backend.c

@@ -1,4 +1,4 @@
-#define _POSIX_C_SOURCE 199309L
+#define _POSIX_C_SOURCE 200112L
 #include <assert.h>
 #include <stdbool.h>
 #include <stdlib.h>