Fixes OSX SDK locations

I was unable to build UV-CDAT on my OSX 10.7 because the CMAKE_OSX_DEPLOYMENT_TARGET variable got forced to 10.8, although I had both 10.7 and 10.8 SDKs in `/Applications/Xcode.app/Contents/Developer/...`

These are the changes I made to the code to be able to build. CMake now looks for 10.6 and 10.7 in both locations.
`CMAKE_OSX_SYSROOT` is what is no longer forced. The `DEPLOYMENT_TARGET` is forced to the lowest available SDK, you are right.