Add MySQL 5.7 symlinks for mysqlclient_r libs

MySQL 5.7 changes lib_r from mysqlclient_r to mysqlclient which causes
packages which don't correctly use mysql_config to determine library
locations to fail due to missing -lmysqlclient_r.

As there are quite a few ports, most of which will require upstream fixes
for now we create symlinks so said ports build without issue.

PR: 206805
Reviewed by: Mahdi Mokhtari (maintainer)
Approved by: koobs (ports)
MFH: 2016Q1
Sponsored by: Multiplay