root: add --with-qt option

The ROOT configure script relies on the existence of the Qt/global.h to enable or not the qt and qtgsi features or not.
This new --with-qt option ensures that, if the qt formula adds a /usr/local/include/Qt symbolic link, the root formula will not by default  try to build ROOT with these features and fail because the qt formula would have been installed without the --with-qt3support option and/or because the check_library of the ROOT configure script is enable to find the Qt frameworks.