So far KWin put a version requirement on Breeze which is the current
version number of KWin. This is technically correct, but distributions
seem to have problems with it. E.g. I noticed twice in Debian testing
that kwin defaults to Plastik which happens when Breeze is not found.
I reported it, it got fixed, now it's again broken.
Thus I think that distributions have problems with the requirement of
building Breeze prior to KWin. As KWin does not depend on the minor
version, let's depend only on the actual required version, which is
5.8 in the 5.8 branch, 5.9 in the 5.9 branch and master.