This option seems to be supported all the way back to at least 10.4, so
enabling it for OS X in general should be fine. If not, someone will
yell.
With no libs statically linked, that's a 3% win in binary size, with
just libevent linked statically, this gives us an advantage of 5% in
terms of binary size, and with libevent and openssl statically linked,
we gain over 18% or over 500KB.
Implements ticket 2915.