linux: lazy call XInitThreads

[Adphi]

This prevents a program that can run as a command line tool from displaying "Unable to init server: Could not connect: Connection refused".

[sqweek]

I looked at this PR when you first opened it and was going to do something with it but I haven't had much bandwidth lately sorry.

Thanks for your work! The change makes sense, but I'd like to take it a bit further before landing it:

1. rename checkStatus() to lazyInit() or similar
2. drop the initSuccess global (ie. panic() within the lazy init block that calls gtk_init_check)