VS 6,7: Refactor local generators to avoid GetSourceFileWithOutput

Use the cmSourceFile returned by AddCustomCommandToOutput instead of
throwing out the return value and looking it up with
GetSourceFileWithOutput.

Source/cmLocalVisualStudio6Generator.cxx

@@ -573,22 +573,20 @@ cmLocalVisualStudio6Generator
 
   // Add the rule with the given dependencies and commands.
   const char* no_main_dependency = 0;
-  this->Makefile->AddCustomCommandToOutput(output,
-                                       depends,
-                                       no_main_dependency,
-                                       origCommand.GetCommandLines(),
-                                       comment.c_str(),
-                                       origCommand.GetWorkingDirectory());
+  if(cmSourceFile* outsf =
+     this->Makefile->AddCustomCommandToOutput(
+       output, depends, no_main_dependency,
+       origCommand.GetCommandLines(), comment.c_str(),
+       origCommand.GetWorkingDirectory()))
+    {
+    target.AddSourceFile(outsf);
+    }
 
   // Replace the dependencies with the output of this rule so that the
   // next rule added will run after this one.
   depends.clear();
   depends.push_back(output);
 
-  // Add a source file representing this output to the project.
-  cmSourceFile* outsf = this->Makefile->GetSourceFileWithOutput(output);
-  target.AddSourceFile(outsf);
-
   // Free the fake output name.
   delete [] output;
 }