ENH: make rc optional

Modules/Platform/Windows-cl.cmake

@@ -29,7 +29,10 @@ ENDIF(CMAKE_GENERATOR MATCHES  "Visual Studio 9")
 
 
 # make sure to enable languages after setting configuration types
-ENABLE_LANGUAGE(RC)
+ENABLE_LANGUAGE(RC OPTIONAL)
+IF(NOT CMAKE_RC_COMPILER_WORKS )
+  MESSAGE(STATUS "Warning: RC not found, this build will not be able to compile resource files.")
+ENDIF(NOT CMAKE_RC_COMPILER_WORKS )
 SET(CMAKE_COMPILE_RESOURCE "rc <FLAGS> /fo<OBJECT> <SOURCE>")
 
 # for nmake we need to compute some information about the compiler