WL#4958: Don't compile libmysql/. Use libmysql_r/ instead.

Since now libmysql project has an own home (Connector/C) the server does not need
to compile and install libmysql (non-thread-safe) version of the client library.
It can be removed from the sources and makefiles.

For backward compatibility, still install a symlink or a copy of
libmysqlclient_r.so as libmysqlclient.so.