- the mysqlc library did link against the mysqlcppconn only when a
  SYSTEM_MYSQL_CPPCONN is used
- mysqlc is an EXTENSIONLIB, mysqlcppconn is PLAINLIBS_NONE
- one N was missing in Extension makefile, and use
  gb_Extension_add_library to put in mysqlcppconn
- manifest.xml, description.xml: update PLACEHOLDERS

Change-Id: If45714f81056a6c1a4cb3380b50cc3dcc0443bb8