determine QQC2 version at build time

Summary:
the qqc2 theme style needs to import the same qqc2 version
than the latest one available in the installed Qt, otherwise
users of QQC2 will only be able to use features available in
Qt 5.7, even if they have a more recent Qt installed.
this will also be needed to conditionally support the icon
property introduced  in Qt 5.10

BUG:386289

Test Plan:
kirigami gallery works with the style installed in this way
it will need a better way to figure out what the available
qqc2 version is

Reviewers: #plasma, rizzitello, davidedmundson, apol

Reviewed By: #plasma, rizzitello, davidedmundson, apol

Subscribers: apol, rizzitello, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D9021