macosx: locate the best newest SDK present on the computer.

With recent versions of XCode, PC/SC headers are hidden inside
the SDK folder inside XCode app, and the global path is not
guaranteed to be present. This allows to build easily from source
without depending on a specific XCode/OSX version.
macosx: locate and target the latest SDK to build against.
macosx: update the minimal building instructions.
Merge pull request #308 from martinpaljak/yosemite

Building on newer OSX versions without xcode command line tools