Makefile: Include OS-specific Makefile if exists

Also only include Makefile.conf if it exists. Check if LIB_EXT is
properly set and boil out if not
Makefile: Add OSX (darwin) specific Makefile

This makefile basically assumes that all 3rd party libraries
are installed (e.g. via homebrew) to /usr/local/lib and they
have been compiled against libstdc++ (not clang's own libc++)