wfcqueue: allow defining CDS_WFCQ_WAIT_SLEEP to override `poll'

Users may want to use alternative sleeping behavior instead of
`poll'.  Make CDS_WFCQ_WAIT_SLEEP a macro which may be defined
before including wfcqueue.h.

This alternative behavior could include logging, performing
low-priority cleanup work, sleeping a shorter/longer interval
or any combination of that.

This will also make integration into glibc easier, as `poll'
linkage causes conformance test failures even when relegated
to an impossible code path:
https://public-inbox.org/libc-alpha/20180801092626.jrwyrojfye4avcis@whir/

Signed-off-by: Eric Wong <normalperson@yhbt.net>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>