Use qmake optimize_full

Qmake now has new config optimize_full that replaces -O2 with -O3. We
should use that in QtWebKit instead of trying to do the same ourselves.

This makes clang now use -O3 like gcc does.

Change-Id: I3db6167833c579bdea941d3ad1b3636cbad62475
Reviewed-by: Michael Bruning <michael.bruning@digia.com>