Merge r1826973 from trunk:

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.

Submitted by: rjung
Reviewed by: rjung, ylavic, jim


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1828741 13f79535-47bb-0310-9956-ffa450edef68