Build fix for OS X 10.9

OS X 10.9 uses libc++ by default and there are several build
issues with Qt 4.8.2 (and possibly later Qt 4.x versions, I haven't
tested)

This commit changes the C++ lib to libstdc++ on Mac.

I'm not sure that this is the right way to go about setting the C++ library to libstdc++ or even whether that is the right thing to do. Would it be better to require a later version of Qt with the libc++ issues fixed instead?