Avoid setting unused variables (-Wself-assign)

Setting a variable and then assigning it to itself avoids
-Wunused-but-set-variable but this specific trick is now caught by
-Wself-assign. Instead, actually use the value or don't bother
assigning it at all:

gdbusauth.c: call g_data_input_stream_read_byte() in void context
gdbusauthmechanismsha1.c: value is actually used
gdbusmessage.c: use consistent preprocessor-token protection
gthreadedresolver.c: skip over bytes in data blob
httpd.c: do something useful with the value

https://bugzilla.gnome.org/show_bug.cgi?id=745723

gio/gthreadedresolver.c

@@ -537,7 +537,6 @@ g_resolver_records_from_res_query (const gchar      *rrname,
   gchar namebuf[1024];
   guchar *end, *p;
   guint16 type, qclass, rdlength;
-  guint32 ttl;
   HEADER *header;
   GList *records;
   GVariant *record;
@@ -587,8 +586,7 @@ g_resolver_records_from_res_query (const gchar      *rrname,
       p += dn_expand (answer, end, p, namebuf, sizeof (namebuf));
       GETSHORT (type, p);
       GETSHORT (qclass, p);
-      GETLONG  (ttl, p);
-      ttl = ttl; /* To avoid -Wunused-but-set-variable */
+      p += 4; /* ignore the ttl (type=long) value */
       GETSHORT (rdlength, p);
 
       if (type != rrtype || qclass != C_IN)