Check clang core C++11 features and headers

Use clang feature checking macros to check for cpp0x's supported
language features and headers. Set corresponding CPP0X_HAS_ preprocessor
symbols based on feature checking results.

Ignore checks for function availability for now. This requires
going through support levels in clang/gnu standard library versions.
NB: Have to worry about clang using its own, or GNU stdc++ library?