Due to the way that the QCC mkspecs are structured, the recent
change to add full optimization support causes the -O2 flag to
be completely dropped from QCC builds in release-with-debuginfo
mode, since the QMAKE_CFLAGS_OPTIMIZE variable is not declared
in any configuration file included by QCC toolchains.  This patch
adds the necessary flags to make the QCC toolchain operate correctly.

Change-Id: I4cd93442d59fae7c92fc5219cddb16f367447203
Reviewed-by: Rafael Roquetto <rafael.roquetto@kdab.com>