WindowsCE: Refactor setting of "WINCE"

Move it to the Platform/Windows module since it is independent of the
compiler used.  While at it, remove redundant "WIN32" setting from
Platform/Windows-MSVC since Plaform/Windows already sets its.

Modules/Platform/Windows-MSVC.cmake

@@ -36,13 +36,10 @@ else()
   set(CMAKE_CL_NOLOGO "/nologo")
 endif()
 
-set(WIN32 1)
-
 if(CMAKE_SYSTEM_NAME STREQUAL "WindowsCE")
   set(CMAKE_CREATE_WIN32_EXE "/entry:WinMainCRTStartup")
   set(CMAKE_CREATE_CONSOLE_EXE "/entry:mainACRTStartup")
   set(_PLATFORM_LINK_FLAGS " /subsystem:windowsce")
-  set(WINCE 1)
 else()
   set(CMAKE_CREATE_WIN32_EXE "/subsystem:windows")
   set(CMAKE_CREATE_CONSOLE_EXE "/subsystem:console")

Modules/Platform/Windows.cmake

@@ -1,5 +1,9 @@
 set(WIN32 1)
 
+if(CMAKE_SYSTEM_NAME STREQUAL "WindowsCE")
+  set(WINCE 1)
+endif()
+
 set(CMAKE_STATIC_LIBRARY_PREFIX "")
 set(CMAKE_STATIC_LIBRARY_SUFFIX ".lib")
 set(CMAKE_SHARED_LIBRARY_PREFIX "")          # lib