Fixes to CMake build to turn off OpenCL build on OS X platforms and properly set CMAKE_OSX_SYSROOT

This PR has two changes:
* OpenCL is disabled by default on all OS X platforms
* I got tired of having to set `CMAKE_OSX_SYSROOT` manually every time I check out OpenMM, so this is now set automatically based on the OS X 10.y version number y.  The 10.7 SDK path should probably be checked.