Make the revision url option in VersionDialog opt-in (defined by build script)

The "About" dialog of Qt Creator can now display a clickable link which opens the git log up to the revision in the web browser.

https://code.qt.io/cgit/qt-creator/qt-creator.git/log/?id=28ce1992172a1d673775086a97b82a8e2ef65ae0

That is a handy feature, especially when dealing with fresh builds to quickly check whether the last patches are included.

However, turning the IDE_REVISION as passed in by the build script into a clickable link (to the qt-creator repo) is currently happening by default. It should better only happen with IDE_REVISION_URL set. Otherwise, builds from forks of Qt Creator (e.g. by distros that have patches on top) that define IDE_REVISION but no own IDE_REVISION_URL will now have a link to the wrong repo.
And perhaps some don't want that link at all.