AutoMoc: Take AUTOMOC_BUILD_DIR into account in depfile

For the Ninja generator and targets that have AUTOMOC_BUILD_DIR set, the
AutoMoc target was always out of date.

That was because the depfile in the AutoMoc build directory was
referencing the wrong timestamp file:
    target_autogen/timestamp: ...dependencies...
instead of
    automoc_build_dir/timestamp: ...dependencies...

Use the relative path of the timestamp file as rule name for the
depfile.  That path is calculated with AUTOMOC_BUILD_DIR taken into
account.

Fixes: #23547

Source/cmQtAutoGenInitializer.cxx

@@ -407,6 +407,7 @@ bool cmQtAutoGenInitializer::InitCustomTargets()
   }
 
   // Common directories
+  std::string relativeBuildDir;
   {
     // Collapsed current binary directory
     std::string const cbd = cmSystemTools::CollapseFullPath(
@@ -424,6 +425,8 @@ bool cmQtAutoGenInitializer::InitCustomTargets()
         cmStrCat(cbd, '/', this->GenTarget->GetName(), "_autogen");
     }
     cmSystemTools::ConvertToUnixSlashes(this->Dir.Build);
+    this->Dir.RelativeBuild =
+      cmSystemTools::RelativePath(cbd, this->Dir.Build);
     // Cleanup build directory
     this->AddCleanFile(this->Dir.Build);
 
@@ -1357,7 +1360,7 @@ bool cmQtAutoGenInitializer::InitAutogenTarget()
         cmStrCat(this->Dir.Build, "/", timestampFileName);
       this->AutogenTarget.DepFile = cmStrCat(this->Dir.Build, "/deps");
       this->AutogenTarget.DepFileRuleName =
-        cmStrCat(this->GenTarget->GetName(), "_autogen/", timestampFileName);
+        cmStrCat(this->Dir.RelativeBuild, "/", timestampFileName);
       commandLines.push_back(cmMakeCommandLine(
         { cmSystemTools::GetCMakeCommand(), "-E", "touch", outputFile }));
 

Source/cmQtAutoGenInitializer.h

@@ -178,6 +178,7 @@ private:
   {
     std::string Info;
     std::string Build;
+    std::string RelativeBuild;
     std::string Work;
     ConfigString Include;
     std::string IncludeGenExp;