Try to work around GCC 4.6's noexcept bugs, again

Try to work around GCC 4.6's noexcept bugs, again

Commit 7315296e9c94c38c842728aed199ac286a3ef174 blacklisted GCC 4.6.0
and 4.6.1 on the grounds that the bug regarding noexcept was fixed in
4.6.2. However, we have evidence of the issue remaining in 4.6.3, so it
looks like it was not the issue that the GCC team fixed.

For that reason, declare noexcept support in GCC 4.6 fatally broken and
enable it only for 4.7. It has been working fine for a month for me on
4.7.1.

Change-Id: Ic5a542235867f20657db12e0e809b83522dc7d23
Reviewed-by: Marc Mutz <marc.mutz@kdab.com>
Reviewed-by: Sean Harmer <sean.harmer@kdab.com>
Reviewed-by: Robin Burchell <robin+qt@viroteck.net>