KDECompilerSettings: Pass -Wvla & -Wdate-time

Summary:
-Wvla: Warn because it's non-standard feature, not supported by MSVC to date
-Wdate-time: Warn because using TIME or DATE prevents reproducible builds

These warnings are being used for *building* Qt itself as well (cf.
qt_common.prf in qtbase)

Test Plan: No new warnings from rebuilding KDE Frameworks

Reviewers: mpyne

Reviewed By: mpyne

Subscribers: mpyne, Frameworks, Build System

Tags: Frameworks, Build System

Differential Revision: https://phabricator.kde.org/D5430