QtAutogen: Do not re-generate AUTORCC outputs on every build

Refactoring in commit v3.6.0-rc1~134^2~10 (Autogen: Split out moc file
generation code to dedicated method, 2016-04-18) removed the
unconditional creation of the `<target>_automoc.cpp` file.  Now it is
generated only when `AUTOMOC` is enabled.  However, if this file is not
created then our internal `GenerateAll` setting is enabled on every
build, causing `AUTORCC` to re-generate its file(s) every time.  Fix the
`GenerateAll` setting to be used only for when autogen settings change.
The old logic was left from when we had only automoc.

Closes: #16413

Source/cmQtAutoGenerators.cxx

@@ -455,8 +455,8 @@ static std::string ReadAll(const std::string& filename)
 
 bool cmQtAutoGenerators::RunAutogen(cmMakefile* makefile)
 {
-  if (!cmsys::SystemTools::FileExists(this->OutMocCppFilenameAbs.c_str()) ||
-      (this->OldCompileSettingsStr != this->CurrentCompileSettingsStr)) {
+  // If settings changed everything needs to be re-generated.
+  if (this->OldCompileSettingsStr != this->CurrentCompileSettingsStr) {
     this->GenerateAll = true;
   }