QtDialog: Avoid linking to Qt4 WinMain when using Qt 5.

Override the QT_QTMAIN_LIBRARY cache variable with a regular
variable in the Qt 5 configuration.

Avoid linking to QT_QTMAIN_LIBRARY at all if CMP0020 is available. If
it is available, it is set at the top of the file, enabling automatic
linking of the correct WinMain library.
Merge topic 'fix-Qt5-windows-build' into next

7ef65550 QtDialog: Avoid linking to Qt4 WinMain when using Qt 5.