mod_ssl, ab: compatibility with LibreSSL. PR 61184.
LibreSSL defines OPENSSL_VERSION_NUMBER = 2.0, but is not compatible with
all of the latest OpenSSL 1.1 API.
Address this by defining MODSSL_USE_OPENSSL_PRE_1_1_API which is true for
anything but OpenSSL >= 1.1 (for now).
Proposed by: Bernard Spil <brnrd freebsd.org>
Reviewed by: ylavic