BUG: keep CMAKE_BACKWARDS_COMP as internal

Source/cmListFileCache.cxx

@@ -164,6 +164,19 @@ bool cmListFile::ParseFile(const char* filename,
           return false;
       }
     }
+    else
+    {
+      // add in the old CMAKE_BACKWARDS_COMPATIBILITY var for old CMake compatibility
+      if (!mf->GetCacheManager()->
+          GetCacheValue("CMAKE_BACKWARDS_COMPATIBILITY"))
+      {
+        mf->AddCacheDefinition
+          ("CMAKE_BACKWARDS_COMPATIBILITY", "2.5",
+           "For backwards compatibility, what version of CMake commands and "
+           "syntax should this version of CMake try to support.",
+           cmCacheManager::INTERNAL);
+      }      
+    }
   }
 
   if(topLevel)