Update XCode SDK search paths

Newer XCodes have moved the SDK path from /Developer/SDKs

Use a suggestion from jorgenisaksson@gmail.com to locate it

osx_sdk_dir is not required to be set. Apple now offers a set
command line tools which do not require this. isysroot is also
not required in newer versions of XCode so only set it when we
are confident in the location.

There remain issues with the iOS configure steps which will be
addressed later

Change-Id: I4f5d7e35175d0dea84faaa6bfb52a0153c72f84b