Removed -Werror for Mac since QT wont build otherwise.

Disabled warnings as errors on mac to address issue #581, since Qt Core won't build on mac due to unused variable warnings in many header files. Looks like:
/usr/local/Cellar/qt/4.8.4/include/QtGui/qmime.h:119:10: warning: private field 'type' is not used [-Wunused-private-field]
    char type;
         ^