Use a test for RbConfig RPATHFLAG
support for mysql-connector-c

check for LIBMYSQL_VERSION, if it exists we are linking against the
connector library, otherwise, use MYSQL_SERVER_VERSION as before

added preprocessor variable to track connector library version
Merge pull request #389 from sodabrew/rbconfig_rpathflag

Use a test for RbConfig RPATHFLAG
Merge pull request #392 from monban/connector-compatability

Per #390 use LIBMYSQL_VERSION where available