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



git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1803396 13f79535-47bb-0310-9956-ffa450edef68