Fix for installation with homebrewed Qt on OS X (10.9)

1) Replaced #include <Qt/qglobal.h> by #include <QtCore/QtGlobal>
For reference: https://github.com/LMMS/lmms/pull/840/files

2) Disabled the lQtgui and lQtCore flags, clang was failing with the following message:
ld: library not found for -lQtGui
clang: error: linker command failed with exit code 1 (use -v to see invocation)