Support for OS X 10.6

Hello,

I managed a couple days ago to build mozart2 on Snow Leopard (10.6) with GCC.
Here is a pull request with related changes.
I still need to work to make the README instructions simpler.

* I checked with `IS_DIRECTORY` to avoid linking with libc++ if not available. This of course only checks for the system include directory (but installing by hand libc++ seems so hard probably very few do it).

* I updated the README.MacOS.md, making two sections for clarity. There is some duplication, but most of it is already present between the main README.md and README.MacOS.md anyway (getting/compiling LLVM).