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/cmpver.c

@@ -22,6 +22,7 @@
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
+#include <strings.h>
 #include <ctype.h>
 
 #include <xbps_api.h>