Fix compiler warning when using OpenSSL 1.1.0.

The old compatibility macro check no longer works,
because those are now actual functions, so an
ifndef is not the correct check.

Backport of r1729435 from trunk.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x-openssl-1.1.0-compat@1755822 13f79535-47bb-0310-9956-ffa450edef68