feature-add osx-clang-libc++ flavour to forumulas

Hey Dan, – first of all: Apothecary is wicked =) !

I've been using apothecary as a way to focus & standardize build efforts for those openFrameworks c++ library dependencies that need recompilation in order to work with clang / libc++.

The commits in this PR add the osx-clang-libc++ flavour to FreeImage and Poco. 

They also make it possible to use the current stable version of Poco with openFrameworks (in cpp98 *and* cpp++), by patching the original poco headers. The patch applies the modifications @arturoc introduced to accomodate for the ofEvent system.

See also: http://github.com/openframeworks-cpp11/openFrameworks