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. This avoids linking with the
Qt 4 version of the WinMain library.
Merge topic 'fix-Qt5-windows-build' into next

61c60b23 QtDialog: Avoid linking to Qt4 WinMain when using Qt 5.