MSVC: Define 'WIN32' for Windows Store and Windows Phone

This was accidentally left out of commit c72f0887 (MSVC: Add default
WindowsPhone and WindowsStore compile flags, 2014-07-28).

Modules/Platform/Windows-MSVC.cmake

@@ -163,6 +163,7 @@ if(WINCE)
     set(CMAKE_C_STANDARD_LIBRARIES_INIT "${CMAKE_C_STANDARD_LIBRARIES_INIT} corelibc.lib")
   endif ()
 elseif(WINDOWS_PHONE OR WINDOWS_STORE)
+  set(_PLATFORM_DEFINES "/DWIN32")
   set(_FLAGS_C " /DUNICODE /D_UNICODE")
   set(_FLAGS_CXX " /DUNICODE /D_UNICODE /GR /EHsc")
   if(WINDOWS_PHONE)