set CMAKE_INSTALL_NAME_DIR for Mac OS

When I install glow in a directory other than `/usr/local/` my binaries can't link against the libraries because CMake removes any rpaths during installation but the Mac OS linker finds the libraries in most cases via absolute paths.

I fixed that by setting the `CMAKE_INSTALL_NAME_DIR`. 

Reference: http://www.cmake.org/pipermail/cmake/2013-February/053551.html