Adapted to GNU GLOBAL 6.2.7.

GNU GLOBAL 6.2.7 is able to parse system.api.php so check version
before choosing gtags label and which files to skip.
Better locating of DRUPAL_ROOT.

Use drush_get_context('DRUSH_SELECTED_DRUPAL_ROOT') instead of using
drush_locate_root().

Forgot to fix this occurrence in d9f88047c44919e9f1eaa3ec88a05deb0854ce00.