doc: update build instructions for OS X

Building Node.js on OS X requires XCode (because node-gyp requires
XCode). Add that information to BUILDING.md.

Additionally, this changes references to `Macintosh` in BUILDING.md to
refer to `OS X`. This is consistent with the way other references are to
operating system families (`Unix`, `Windows`) and not brand names or
hardware architectures.

PR-URL: https://github.com/nodejs/node/pull/6309


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Johan Bergström <bugs@bergstroem.nu>