Enable QT_STRICT_ITERATORS regardless of Qt version

If anyone should run into problems, they can disable strict iterators
using a global cache variable:

$ cmake DENABLE_QT_STRICT_ITERATORS=OFF [...]
2 jobs for qt-strict-iterators in 6 minutes and 4 seconds (queued for 2 seconds)
latest
Status Job ID Name Coverage
  Build
passed #24013
build

00:04:03

passed #24014
build_clang_libcpp

00:04:25