Closes ticket 20303.
The LIBRESSL_VERSION_NUMBER check is needed because if our openssl
is really libressl, it will have an openssl version number we can't
really believe.
LibreSSL doesn't use OpenSSL_version (it uses the older SSLeay_version
API), but it reports a major version number as 2 in
OPENSSL_VERSION_NUMBER. Instead of fudging the version check, for now,
let's just check if we're using LibreSSL by checking the version number
macro exists, and use compatibility defines unconditionally when we
detect LibreSSL.
