boost: Fix C++11 build.

* Use default darwin toolset.
  - Used to set toolset as clang in C++11 build, which will override
    darwin settings and fail to increase template depth which is
    required for some of the libraries such as Boost.Log.
* Remove unnecessary "-fPIC" build flag.

Closes #22942.
Merge branch 'master' of http://github.com/mxcl/homebrew