Don't build code with _GNU_SOURCE, use _XOPEN_SOURCE=500 instead.

_GNU_SOURCE is only now for xbps_xasprintf() if vasprintf is found,
and only for <stdio.h>, it's unset immediately once the header is included.

lib/util.c

@@ -23,7 +23,13 @@
  * THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  */
 
-#include <stdio.h>
+#ifdef HAVE_VASPRINTF
+# define _GNU_SOURCE	/* for vasprintf(3) */
+# include <stdio.h>
+# undef _GNU_SOURCE
+#else
+# include <stdio.h>
+#endif
 #include <stdbool.h>
 #include <stdlib.h>
 #include <stdarg.h>