build: Add -Werror=undef by default, fix fallout

The main thing here is that a ton of stuff has happened in gnulib since we
imported `parse-datetime.y`. I cherry-picked a little bit of it, but that
upstream doesn't seem to build with `-Wundef`, so I just deleted some hunks.

(Note I reindented the warnings consistently)

Update submodule: libglnx

Closes: #1320
Approved by: jlebon

src/libostree/ostree-enumtypes.c.template

@@ -18,9 +18,7 @@
  * Boston, MA 02111-1307, USA.
  */
 
-#ifndef _GNU_SOURCE
-#define _GNU_SOURCE
-#endif
+#include "config.h"
 #include <string.h>
 
 #include "ostree-enumtypes.h"