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
a05bf133ae