by default webkit adds -O2 after the user's CFLAGS, so any other -O setting will be overridden. eg. -O3 -march=core2 -fomit-frame-pointer -pipe -O2 -MT WebCore/page/libWebCore_la-WorkerNavigator.lo -MD -MP -MF WebCore/page/.deps/libWebCore_la-WorkerNavigator.Tpo -c -o WebCore/page/libWebCore_la-WorkerNavigator.lo `test -f 'WebCore/page/WorkerNavigator.cpp' || echo './'`WebCore/page/WorkerNavigator.cpp adding --disable-optimizations to econf forces -O0, so to fix this we'll have to patch configure.ac.
Created attachment 182097 [details, diff] webkit-gtk-0_p40220-CFLAGS.patch
This has been fixed in upstream trunk, will go into 1.1.8
Wasn't fixed upstream, bug fixed in the ebuild for now