QDoc: Make QDoc handle QT_VERSION gracefully

See: https://codereview.qt-project.org/#change,46273,patchset=3

There will be a QT_VERSION variable to handle the naming of files during generation time.

QDoc will stop execution if that QT_VERSION variable is not defined, meaning the only way to build the documentation is through 'make docs' or when the QT_VERSION is passed in as an argument.

What QDoc can do is handle the QT_VERSION variable gracefully. The QT_INSTALL_DOCS is handled gracefully already.