When Emacs is blocked in condition-wait, then C-g doesn't seem to work.
E.g. Emacs hangs and pressing C-g has no apparent effect when executing
the attached file with:

  emacs -Q -l deadlock.el -f deadlock

Mabye sys_cond_wait could call pthread_cond_timedwait with a fairly long
timeout, say one second, and repeatedly check if C-g was pressed.