Removed most of Darwin-specific configuration.

* Pass CMAKE_CXX_FLAGS to cmake subprocesses instead.
Improve the build instructions for OS X

* Add instructions for using the Homebrew Tap
* Explain a bit about libc++
* Mention the new flags which need to be passed
* Reorder options so they are in the same order
Merge pull request #124 from eregon/osx

Remove most of Darwin-specific configuration