KDevelop does not build with older Qt

Hi, 

we claim we support building with Qt 5.9 but that is unfortunately not the 
case in practice. A friend of mine tried to build KDevelop with Qt 5.11 which 
fails.
We have no CI coverage for anything pre Qt 5.15 and  I think this shows that 
no Distro or anyone else actually builds KDevelop with such old Qt versions. 
Either way we would have been notified of build failures.

I see three options
1) Be careful when committing to no break older Qt
2) Build ourselves against the minimum required versions
3) Bump the Qt version to what is actually tested

It's clear that 1) did not work otherwise I would not be writing this email ;D

Regards,
David