Compile libraries under clang on Mac

With these changes I can run `make` on my Mac and build ann, sparsehash, and contraction_hierarchies with Apple's `clang` compiler. I still cannot build the pyaccess extension, though, because `clang` does not support `dllexport` and `dllimport` used there. If we want to be able to build this without GNU gcc we'll have to take those out somehow.