Change misleading error message in QDBusReply

"no signature" is misleading and even led me to re-check the code to see
if it meant that the reply failed and no signature could be obtained.
Saying that the signature was obtained but is empty is better.

Change-Id: I1381cf53b334798125d36db0934105d15b63b84f
Reviewed-by: Frederik Gladhorn <frederik.gladhorn@theqtcompany.com>
Configure: Fix detection of GCC 5

$ gcc-5 -dumpversion
5

No dots.

Change-Id: I5dd547f257718c981e7be64fca3eec980136ba3e
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@theqtcompany.com>