Added PKGBUILD for compiling with GCC 4.6 compatibility.
UseCorrade: add `-W...` to CORRADE_CXX_FLAGS, not CMAKE_CXX_FLAGS.
UseCorrade: check also for compiler version when adding `-W` flag.

With this change it's possible to use UseCorrade from `master` branch
with library otherwise compiled from `compatibility` branch.
Merge branch 'master' into compatibility