Support building against Qt v4.7.2 using "modern" GCC versions

With "modern" GCC versions we got some new warnings that, due to the
-Werror, were treated as errors. Just kill the -Werror flag.

Change-Id: I3947fde8b56dab8f3810366aa27dc25967ba5b10
Reviewed-by: David Faure <david.faure@kdab.com>