Improve libmysqlclient exists check in wscript
Improve libmysqlclient exists check in wscript

beni55/node-mysql-libmysqlclient,Sannis/node-mysql-libmysqlclient,beni55/node-mysql-libmysqlclient,beni55/node-mysql-libmysqlclient,Sannis/node-mysql-libmysqlclient,Sannis/node-mysql-libmysqlclient,Sannis/node-mysql-libmysqlclient,beni55/node-mysql-libmysqlclient,beni55/node-mysql-libmysqlclient