[Qt] Make qt.cmake Windows-friendly

- Do not build headless view.
- Add no-op thread implementation.

platform/qt/qt.cmake

@@ -96,13 +96,17 @@ if (MASON_PLATFORM STREQUAL "osx" OR MASON_PLATFORM STREQUAL "ios")
         PRIVATE "-framework Foundation"
         PRIVATE "-framework OpenGL"
     )
-else()
+elseif (CMAKE_HOST_SYSTEM_NAME STREQUAL "Linux")
     list(APPEND MBGL_QT_FILES
         PRIVATE platform/default/thread.cpp
     )
     list(APPEND MBGL_QT_LIBRARIES
         PRIVATE -lGL
     )
+elseif (CMAKE_HOST_SYSTEM_NAME STREQUAL "Windows")
+    list(APPEND MBGL_QT_FILES
+        PRIVATE platform/qt/src/thread.cpp
+    )
 endif()
 
 add_custom_command(