Enable pyqt to build with libc++ on Mavericks

This patch enables pyqt to build with libc++ on Mavericks. The prerequisite though is a version of Qt building with libc++. Currently such changes are under code review upstream: https://codereview.qt-project.org/#change,70930, and so a patched qt formula is also required for this change to work, see here: https://github.com/jensenb/homebrew/blob/qtFixMavericks/Library/Formula/qt.rb

Once the Qt issues are resolved upstream I will update the formula.