build: do not remove definition of ENODATA, after all

* src/system.h (ENODATA): Restore definition.
gnulib defines it only on native Windows systems, so removing our
definition would have provoked build failure on systems that use it,
like FreeBSD.  Reported by Bruno Haible in
http://thread.gmane.org/gmane.comp.lib.gnulib.bugs/28739/focus=28795
* gnulib: Update to latest, to get new ENODATA-exempting maint.mk rule.

src/system.h

@@ -76,6 +76,14 @@ you must include <sys/types.h> before including this file
 
 #include <string.h>
 #include <errno.h>
+
+/* Some systems don't define this; POSIX mentions it but says it is
+   obsolete.  gnulib defines it, but only on native Windows systems,
+   and there only because MSVC 10 does.  */
+#ifndef ENODATA
+# define ENODATA (-1)
+#endif
+
 #include <stdbool.h>
 #include <stdlib.h>
 #include "version.h"