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.
I think we don't want to do that as it may not be liked by some others. I think the correct patch should do this:

1. Find out what SDK versions are present 
2. Initially set the TARGET to newest one
3. If over-ridden by the user, then respect that assuming that target is found earlier or else throw an error.