Implement pthread_cond_broadcast on Windows

Implement pthread_cond_broadcast on Windows

See http://www.cse.wustl.edu/~schmidt/win32-cv-1.html, section "The
SignalObjectAndWait solution". But note that this implementation does not
use SignalObjectAndWait (which is needed to achieve fairness, but we do
not need fairness).

Note that our implementations of pthread_cond_broadcast and
pthread_cond_signal require that they are invoked with the mutex held that
is used in the pthread_cond_wait calls.

Signed-off-by: Johannes Sixt <j6t@kdbg.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>