add proper compiler switches for clang

This adds GCC-like switches for warnings/C++11 when compilig with Clang.
I disabled the overloaded virtual warnings as the Ogre headers have quite a lot of them.

With the new settings there are quite a few warnings, mostly (harmless) struct/class mismatches in the nif-files but also at least one real bug that I saw skimming through them. I'll make another pull request after fixing these.