XBox 360 Support for Mac OS

Adds support for XBox 360 controllers to the current Mac OS gamepad
support.  Also adds null checks to one of Gamepad's constructors.
Change header and lib search paths in xcode project.

Relative paths to external-deps and src folders were assuming that the
repo was cloned to a folder named "Gameplay", and not e.g.
"Gameplay-Next" or "MyGameplayFork".
Merging with upstream/next
Merge pull request #844 from ablake/next

XBox 360 controller support for Mac OS