xquartz: Ensure that NSRunAlertPanel() is run on the main thread

Fixes: https://github.com/XQuartz/XQuartz/issues/30
Signed-off-by: Jeremy Huddleston Sequoia <jeremyhu@apple.com>
(cherry picked from commit 520e7a13)
3 jobs for server-1.20-branch in 7 minutes and 23 seconds (queued for 1 second)