cmake-gui: Add build option to use Qt5 windows plugin statically

This will enable builds against a static Qt5 on Windows.

Source/QtDialog/CMakeLists.txt

@@ -33,6 +33,12 @@ if (Qt5Widgets_FOUND)
       PROPERTY COMPILE_DEFINITIONS USE_QXcbIntegrationPlugin)
   endif()
 
+  if(CMake_QT_STATIC_QWindowsIntegrationPlugin_LIBRARIES)
+    list(APPEND CMake_QT_LIBRARIES ${CMake_QT_STATIC_QWindowsIntegrationPlugin_LIBRARIES})
+    set_property(SOURCE CMakeSetup.cxx
+      PROPERTY COMPILE_DEFINITIONS USE_QWindowsIntegrationPlugin)
+  endif()
+
   # We need to install platform plugin and add qt.conf for Qt5 on Mac and Windows.
   # FIXME: This should be part of Qt5 CMake scripts, but unfortunatelly
   # Qt5 support is missing there.
@@ -65,7 +71,7 @@ if (Qt5Widgets_FOUND)
       install(FILES "${CMAKE_CURRENT_BINARY_DIR}/qt.conf"
         DESTINATION "${CMAKE_INSTALL_PREFIX}/Resources"
         ${COMPONENT})
-    elseif(WIN32)
+    elseif(WIN32 AND NOT CMake_QT_STATIC_QWindowsIntegrationPlugin_LIBRARIES)
       install_qt5_plugin("Qt5::QWindowsIntegrationPlugin" QT_PLUGINS)
       file(WRITE "${CMAKE_CURRENT_BINARY_DIR}/qt.conf"
         "[Paths]\nPlugins = ../${_qt_plugin_dir}\n")

Source/QtDialog/CMakeSetup.cxx

@@ -44,6 +44,10 @@ static void cmAddPluginPath();
 Q_IMPORT_PLUGIN(QXcbIntegrationPlugin);
 #endif
 
+#if defined(USE_QWindowsIntegrationPlugin)
+Q_IMPORT_PLUGIN(QWindowsIntegrationPlugin);
+#endif
+
 int main(int argc, char** argv)
 {
   cmsys::Encoding::CommandLineArguments encoding_args =