mod_md OpenSSL version requirement 1.0.0

[Rainer Jung]

It seems mod_md (trunk and 2.4, currently identical) needs OpenSSL 1.0.2 
(for ASN1_TIME_diff), but with a small change (using the already 
existing LIBRESSL alternative code) it only needs 1.0.0.

Since we still support 0.9.8a+ for 2.4.x and trunk, I think we need to 
add a version check to modules/md/config2.m4, maybe by allowing the 
requested version as an argument to APACHE_CHECK_OPENSSL which is 
already used there. Currently APACHE_CHECK_OPENSSL uses a hard-coded 
0.9.8a check.

Regards,

Rainer