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/tools/http_common.c

@@ -35,8 +35,6 @@
 
 #include "config.h"
 
-#define _GNU_SOURCE 1
-
 #include <stdlib.h>
 #include <stdio.h>
 #include <string.h>