client library:
- implemented 'check_license' function


include/errmsg.h:
  WL #1510: new error code
include/mysql_version.h.in:
  WL #1510: fallback define for LICENSE
libmysql/errmsg.c:
  WL #1510: error message to print in case when client and server license 
  mismatch.
libmysql/libmysql.c:
  WL#1510: implementation of 'check_license' function
sql/set_var.cc:
  removed unused variable 'license'