Port mkspec QT_QPA_DEFAULT_PLATFORM mechanism to CMake

In src/gui/configure.pri there's:

defineTest(qtConfTest_qpaDefaultPlatform) {
    name =
    !isEmpty(config.input.qpa_default_platform): name = $$config.input.qpa_default_platform
    else: !isEmpty(QT_QPA_DEFAULT_PLATFORM): name = $$QT_QPA_DEFAULT_PLATFORM
.......
}

Note the check for the QT_QPA_DEFAULT_PLATFORM qmake variable.
This is set in various mkspecs/devices files.

We currently lack this behaviour in the CMake port, which means most cross-compiling Linux targets get 'xcb' as the default qpa plugin, despite xcb most likely not being available.

We should find a way to specify and port this mechanism to CMake.

A workaround when launching applications is to set the QT_QPA_PLATFORM env var.