mod_md: Fix compilation with OpenSSL before version 1.0.2.

Symbol ASN1_TIME_diff is only available for 1.0.2+,
but luckily alternative code we can use is already
available, originally written for the LibreSSL case.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1826973 13f79535-47bb-0310-9956-ffa450edef68