RC: Do not override MinGW Makefiles generator preference

Update logic added in commit 957c2aac (RC: Simplify selection of
resource compiler based on C/C++ toolchain, 2015-05-07) to avoid
overriding CMAKE_GENERATOR_RC.  The MinGW and MSYS Makefiles
generators use it to select a windres next to the compiler even
if it is not in the PATH.

Modules/Platform/Windows-GNU.cmake

@@ -138,7 +138,7 @@ macro(__windows_compiler_gnu lang)
     endforeach()
   endif()
 
-  if(NOT CMAKE_RC_COMPILER_INIT)
+  if(NOT CMAKE_RC_COMPILER_INIT AND NOT CMAKE_GENERATOR_RC)
     set(CMAKE_RC_COMPILER_INIT windres)
   endif()