Fix (wrong) clang 3.4 warning (happens on Mavericks)

I just update to OS X 10.9 Mavericks.
Apart from that warning, it cleanly builds, but I had to change a couple things:
* Reinstall the command-line tools with `xcode-select --install` (Homebrew asks for it whenever you use it)
* Reinstall boost with C++11 support (it was producing warnings about incompatible integral types): `brew install boost --c++11`.
* In the CMake configuration, the path to C++11 headers changed from `/usr/lib/c++/v1` to `/Library/Developer/CommandLineTools/usr/lib/c++/v1`. (Without the command line tools, they are available in `/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/lib/c++/v1` and other very long paths which are not supposed to be used). This is actually needed only for the custom LLVM/Clang which does not know the right system paths. The system clang knows them and needs no hint.
I shall update the README in #124.

See the commit description for details about the warning.

@sjrd @sjmackenzie Could you merge this soon?