Don't enable QT_STRICT_ITERATORS on Windows.

Summary:
Strict iterators can't be used on Windows, they lead to a link error
when application code iterates over a QVector<QPoint> for instance, unless
Qt itself was also built with strict iterators.
See example at https://bugreports.qt.io/browse/AUTOSUITE-946

Technically this would be fine for mingw, but not for MSVC neither
clang-cl (which also uses the MSVC ABI). I think it's fine to just
disable it for all Windows compilers, since any iterators misuse
will be detected on Unix anyway.

Test Plan: None, I'm relying on Volker's findings.

Reviewers: vkrause, dvratil

Reviewed By: dvratil

Subscribers: apol, kde-frameworks-devel, kde-buildsystem

Tags: Frameworks, Build System

Differential Revision: https://phabricator.kde.org/D21314