Fix MathGL when building with Qt

Fix for #897.

MathGL changed the Qt option from `enable-qt` to `enable-qt4` and `enable-qt5` to add Qt 5 support. I corrected the argument passed to CMake to fix this.

To add Qt 5 support, I added an option argument (parsed by `build.with_qt?`). To install with Qt 4, pass `--with-qt=4`; for Qt 5 use `--with-qt=5`.