Use nomlib's modularized find module

* nomlib's new find module includes automatic dependency
management for its external libraries, freeing us from the burden of
manually setting it up -- w00t!

* platform.cmake: Clean up how the OSX SDK is set; passing
-D CMAKE_OSX_DEPLOYMENT_TARGET=<ver> takes care of everything that we
had to do manually before, with much cleaner syntax.
Remove third-party dir
README.md: Update build info for OSX, Windows