proposal: make plasma *not* build with Qt 5.8.0

[Marco Martin]

Hi all,
I'm seeing lately many new crash reports in bugzilla which are with Qt 5.8.0, 
and some even backtraces that i never seen.
Knowing the big problems that Qt 5.8.0 had, I've come to thinking that to make 
even more obvious that 5.8.0 is discouraged, to make cmake fail in plasma-
workspace with exactly that version (with 5.8 from branch or 5.9 still fine) so 
if a distribution really wants that combination would have to patch it, making 
it obvious it's their problem, and not supported.

What do you think?

-- 
Marco Martin