Force gmap to properly respect a custom marker path (as stored in gmap_markerfiles variable)

Currently gmap permits an administrator to specify a custom path to the markerfiles. This path is stored in a Drupal variable called gmap_markerfiles. However, while the _gmap_get_icondata() and _gmap_get_marker_titles() functions in gmap_markerinfo.inc respect this value, the gmap_regenerate_markers() function that writes the path into the generated js file always writes a path to the gmap module markers folder. This tiny patch fixes the problem.