Slackware install docs to different folder, which wasn't handled
by file_contents test properly. E.g. on Redhat/Debian docs are
expected to be in /usr/share/doc/mariadb-server-(version), but
on Slackware they are in /usr/doc/mariadb-(version).

For details see Slackware mariadb build script:
http://slackbuilds.org/slackbuilds/14.0/system/mariadb/mariadb.SlackBuild