Fix install directory on Mac OS X

Due to [request 2362](https://github.com/D-Programming-Language/phobos/pull/2362), Phobos lib is installed into `$(INSTALL_DIR)/lib64` on Max OS X 64bit systems
but it should be installed into `$(INSTALL_DIR)/lib`.
This pull request fixes this issue on Max OS X systems.

Related issue on druntime: [request 910](https://github.com/D-Programming-Language/druntime/pull/910)