mail/dspam: fix configure with modern MariaDB

1. Configure fails with error message:

checking for MySQL client version >= 5.0.0... no
configure: error: Required version of libmysqlclient not found

It check "mysql_get_client_info() > 5", but modern MariaDB return
Connector/C's version 3.x instead of MariaDB version.
How to get the correct version found here:
https://github.com/pali/DBD-MariaDB/commit/9743bec52db5e8f1beb2b31e4a55d6ea1a4edcdd

While here:

2. remove GNU_CONFIGURE_MANPREFIX
3. fix make check-plist if build with MYSQL option ON only:

> Checking for items in STAGEDIR missing from pkg-plist

Error: Orphaned: @dir lib/%%DSPAM_OWNER%%

PR: 279253
Approved by: danny@dannywarren.com (maintainer, timeout 17 days)