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/CMakeLists.txt

@@ -1,3 +1,5 @@
+include_directories(${MARIADB_CONNECTOR_INCLUDE})
+
 add_subdirectory(core)
 add_subdirectory(modules)
 add_subdirectory(inih)