Compile Qt5 frontend with -DQT_STRICT_ITERATORS
By default, with Qt containers it's sometimes possible to assign non-const iterators to const-iterators. In that situations we may waste resources due to needless deep copy during detach from implicitly shared data.
QT_STRICT_ITERATORS helps us getting aware by breaking the build.
The check for Qt < 5.3 and WIN32 is due to QTBUG-29608, which broke Windows builds. It's fixed in Qt 5.3.