etc flags which might not be ported to CMake, which leads to different compilation flags when building with CMake.
I think the easiest would be to pick some .cpp file in Gui like image\qpixmapcache.cpp, and compare the compile and linker flags between a qmake and equivalent CMake platform configuration
Add the appropriate missing definition flags to QT_DEFAULT_PLATFORM_DEFINITIONS in QtBuild.cmake.
Afaik these are public mkspec flags which are used also when compiling user code (customer code that uses Qt).
Public mkspec compile and link flags can be set on the Platform target in cmake/QtBaseGlobalTargets.cmake.
There might also be some missing flags that come from private qmake files like qt_module.prf, qt_common.prf, qt_plugin.prf, etc
The equivalent CMake targets are PlatformCommonInternal / PlatformModuleInternal / PlatformPluginInternal / PlatformToolInternal in cmake/QtInternalTargets.cmake.