Silence "defined but not used" compiler warnings

when building against OpenSSL 0.9.8a.

Backport of r1755874 from trunk.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x-openssl-1.1.0-compat@1755875 13f79535-47bb-0310-9956-ffa450edef68