Modified PyQT and Qwt formulae to work on OS X 10.9 Mavericks.

In both formulae, a check to see if Mavericks is the host operating
system is included to modify the -spec flag given to qmake, which
defaults to macx-g++, which builds against libstdc++ rather than libc++.

This default configuration for qmake causes all new compiled code linked
against libc++ built code (e.g. Qt HEAD, the current workaround for
getting Qt to build on Mavericks) to fail.

This fixes the issues with PyQwt found in #23701 after recompiling Qt4
per the notes in #23793.