OSX: warn when MASON_XCODE_ROOT does not exist

[springmeyer]

It looks possible for the value returned by xcode-select -p to not exist. In this case we should stop the build or warn the user of the potential problem.

refs

mason/mason.sh

Line 12 in 6aa9591

MASON_XCODE_ROOT=$("xcode-select" -p)

/cc @boblannon I think you hit this.

[boblannon]

just to clarify, it wasn't null. it was:

$ xcode-select -p
/Library/Developer/CommandLineTools

...and remained that way even after installing XCode. To get it set correctly, I needed to run this after installing XCode:

sudo xcode-select -s /Applications/Xcode.app/Contents/Developer