Only build Qt if necessary

By default, kdesrc-build --initial-setup writes a ~/.kdesrc-buildrc file that includes configuration bits (the qtdir global setting and two Qt-related include paths) that cause Qt to get built unconditionally, regardless of whether or not the distro-provided Qt version is old enough to warrant this.

It would be nice if kdesrc-build only built Qt when the distro-provided version would otherwise be too old to compile something that it's been told to compile.

I have seen a number of people tripped up by this recently, and we now have this quirk documented at https://community.kde.org/Get_Involved/development#Set_up_kdesrc-build. It would be nice to be able to remove this information and have this case handled in code so people don't encounter issues in the first place. :)