Makefile: fix to OSX version detection; remove forced linking to 10.5 SDK when using 10.6 and above (can override with OSX_VERSION flag)
Makefile: fix to OSX version detection; remove forced linking to 10.5 SDK when using 10.6 and above (can override with OSX_VERSION flag)

AlloSphere-Research-Group/AlloSystem,AlloSphere-Research-Group/AlloSystem,AlloSphere-Research-Group/AlloSystem,AlloSphere-Research-Group/AlloSystem,AlloSphere-Research-Group/AlloSystem