make automoc work when using ccmake via PATH (#12551)

In ccmake, cmake::GetCMakeCommand() returns "/cmake" when ccmake
was started from the PATH, i.e. by simply running "ccmake"

Alex

Source/cmQtAutomoc.cxx

@@ -67,7 +67,7 @@ void cmQtAutomoc::SetupAutomocTarget(cmTarget* target)
   targetDir += ".dir/";
 
   cmCustomCommandLine currentLine;
-  currentLine.push_back(makefile->GetCMakeInstance()->GetCMakeCommand());
+  currentLine.push_back(makefile->GetSafeDefinition("CMAKE_COMMAND"));
   currentLine.push_back("-E");
   currentLine.push_back("cmake_automoc");
   currentLine.push_back(targetDir);