ZLIB_VERNUM isn't defined in some zlib versions, so this patch does a proper
linking test in autoconf to see whether deflateBound exists in zlib. Also,
setting NO_DEFLATE_BOUND will also work for folk not using autoconf.
Signed-off-by: David Symonds <dsymonds@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>