cmAddCustomCommandCommand: use `cmStrCat`

Source/cmAddCustomCommandCommand.cxx

@@ -189,8 +189,8 @@ bool cmAddCustomCommandCommand(std::vector<std::string> const& args,
       } else if (copy == keyDEPFILE) {
         doing = doing_depfile;
         if (!mf.GetGlobalGenerator()->SupportsCustomCommandDepfile()) {
-          status.SetError("Option DEPFILE not supported by " +
-                          mf.GetGlobalGenerator()->GetName());
+          status.SetError(cmStrCat("Option DEPFILE not supported by ",
+                                   mf.GetGlobalGenerator()->GetName()));
           return false;
         }
       } else if (copy == keyJOB_POOL) {