QtAutogen: Add comment explaining why rcc cannot use PRE_BUILD

Source/cmQtAutoGenerators.cxx

@@ -440,6 +440,9 @@ bool cmQtAutoGenerators::InitializeAutogenTarget(cmTarget* target)
               this->ListQt4RccInputs(sf, depends);
               }
 #if defined(_WIN32) && !defined(__CYGWIN__)
+            // Cannot use PRE_BUILD because the resource files themselves
+            // may not be sources within the target so VS may not know the
+            // target needs to re-build at all.
             usePRE_BUILD = false;
 #endif
             }