export(): Check targets exist at configure-time (#14608)

Commit 66b290e7 (export(): Process the export() command at generate
time., 2012-10-06 ) refactored export() so that it could evaluate
strings at generate-time. This was intended for evaluating target
properties, but that commit also removed a check for target
existence at configure-time. Restore that check and add a test for
this case.

Source/cmExportCommand.cxx

@@ -135,6 +135,14 @@ bool cmExportCommand
         return false;
         }
       }
+    else
+      {
+      cmOStringStream e;
+      e << "given target \"" << *currentTarget
+        << "\" which is not built by this project.";
+      this->SetError(e.str().c_str());
+      return false;
+      }
     }
 
   cmGlobalGenerator *gg = this->Makefile->GetLocalGenerator()