Get rid of asprintf and _GNU_SOURCE define

_GNU_SOURCE was even in some files where it was not needed at all
(http*).

Let's replace asprintf() with g_strdup_printf().

src/ui/mucwin.c

@@ -37,7 +37,6 @@
 #include "config.h"
 
 #include "ui.h"
-#define _GNU_SOURCE 1
 
 #include <string.h>
 #include <assert.h>