RC: Add platform-specific preprocessor definitions (#15404)

In Platform/Windows-MSVC the C and CXX flags are initialized to
contain preprocessor definitions describing the platform.  On
WinCE platforms this may not be just -DWIN32.  This information
may be important to RC sources too, so add such preprocessor
definitions to the default RC flags.

Suggested-by: Gunnar Roth <gunnar.roth@gmx.de>

Modules/Platform/Windows-MSVC.cmake

@@ -271,5 +271,9 @@ macro(__windows_compiler_msvc lang)
   set(CMAKE_${lang}_FLAGS_MINSIZEREL_INIT "/MD /O1 /Ob1 /D NDEBUG")
   set(CMAKE_${lang}_LINKER_SUPPORTS_PDB ON)
 
+  if(NOT CMAKE_RC_FLAGS_INIT)
+    set(CMAKE_RC_FLAGS_INIT "${_PLATFORM_DEFINES} ${_PLATFORM_DEFINES_${lang}}")
+  endif()
+
   enable_language(RC)
 endmacro()