Add note on Clang vs Apple Clang

Know that Apple uses its own version numbers for clang, which will
appear in compiler macros rather than the LLVM svn version number.
Make a note about this in the clang configuration.
Emit warning when not using GNU or Clang

No support/knowledge of C++11 support in compilers other than GNU and
Clang. Emit a warning to note this, but do not fatal_error as we would
like to support these going forward (though ideally the compilers
support C++11 themselves!).
Use clang version to select C++ stdlib features

Encode clang version in single integer as is done for GNU C++. Use
knowledge of Apple's clang version mapping to LLVM to convert reported
version on Apple to LLVM equivalent. Note that this may not work for
non-Apple installs of Clang (e.g. Macports).

Assume that clang version >= 3.2 will enable all relevant features,
defining the symbols in this case. Note that there are compile
problems when not using libc++. Not clear if this is cpp0x errors
or stdlibc++ incompatibilities.
Add futher clang/gnu checks

Discovered a knarliness with Clang - enabling -std=c++11 is not enough,
you also need a compliant Standard C++ library. On the latest Xcode on
Mac, all that should be needed is -std=c++11 -stdlib=libc++. Leaving
out the latter results in errors because the shipped libstdc++ does
not support some features.

Add some further notes and attempts at checks. The adaption by cpp0x
should handle the use of an older libstdc++, but needs further checking.
Look at how Boost.Config does compiler and library detection.