I have encountered the following problems while trying to build Qt 5.9.2 for Windows from Linux using cross mingw:

1. qtbase/mkspecs/win32-g++/qmake.conf constains the following block: equals(QMAKE_HOST.os, Windows) {
   QMAKE_LINK_OBJECT_MAX = 10
   QMAKE_LINK_OBJECT_SCRIPT = object_script
   }, but these 2 variables have to be assigned even if I cross compile from Linux, otherwise generated Makefiles can't be used to build some static libraries (qt harfbuzz for instance), so I had to comment out the condition.
2. qtactiveqt/src/tools/idc/idc.pro, I had to comment out first two lines, since idc contains windows specific code and can not be built for linux.
3. qtmultimedia/src/plugins/common/evr/evrd3dpresentengine.cpp, line 52, had to replace qtgui with QtGui, since linux file systems are case sensitive.