into the server repository

This patch adds client windows authentication plugin code to the client 
library libmysql (only on Windows platform). The plugin is compiled into 
the library and added to the list of built-in plugins. This way clients 
should be able to connect to a server which uses windows authentication 
plugin even as an SQL user which uses such authentication.

Note: this makes the client library to depend on Secur32 Windows system
library. When building clients, they must be linked against Secur32. 
Command mysql_config --libs correctly lists Secur32 as a required 
dependency.