libcxx port

Hi!
With these changes openmw builds and seems to run fine with clang/libc++ on current testing Gentoo.

I have not done extensive testing with other platforms (I don't have windows or macos available, and rebuilding dependencies for a different stdlib is a hassle), but the build seems to work with gcc-4.8.0. I do not expect to have broken anything, though.

Several approaches for selecting tr1/c++11 are possible, if this one isn't palatable just tell me and I can make one more to your liking. I couldn't find a way to directly check which stdlib is in use either in preprocessor (without messily including other headers) or in cmake, but I think this functionality test is more robust and future/c++11-proof.