Handle dir_config and with_config mysql more strictly, add rpath more often

Add notices, too, such as:

```
$ bundle exec rake compile
-----
Using mysql_config at /opt/local/bin/mysql_config
-----
checking for mysql.h... yes
checking for errmsg.h... yes
checking for mysqld_error.h... yes
-----
Setting rpath to /opt/local/lib/mysql56/mysql
-----
```

```
$ bundle exec rake compile -- --with-mysql-config=/bad/path
-----
Cannot find mysql_config at /bad/path
-----
```

FIXME: this isn't working as I expected (should fail gracefully)
```
$ bundle exec rake compile -- --with-mysql-dir=/bad/path
Using --with-mysql-dir=include
-----
Using mysql_config at /opt/local/bin/mysql_config
-----
```