Added automatic Xcode location determination to better support Xcode beta releases

[TahoeJimbo]

I changed build.sh to use xcode-select -p to determine the location of the toolchain and SDKs. The error message printed by xcode-select helps point folks in the right direction, and it makes juggling Xcode beta versions a little easier.

Feel free to pull it in if you think it's useful.

-Jimbo

[zaichang]

I was about to do the same but saw that you've already changed the script to use xcode-select. Second the pull request, with an update to SDK version to 8.1