Link against Connector-C.

Now, qc_mysqlembedded is linked against MySQL's embedded library,
and MaxScale itself against Connector-C.

So, in order to build MaxScale, Connector-C must be installed.
This has been tested with Connector-C 2.2.1.

The build variable MYSQLCLIENT_LIBRARIES is no longer used.

server/modules/filter/mqfilter.c

@@ -80,7 +80,6 @@
 #include <query_classifier.h>
 #include <spinlock.h>
 #include <session.h>
-#include <plugin.h>
 #include <housekeeper.h>
 
 MODULE_INFO info =