Patch to make mysqlpp build correctly on MSVC2010

[Ilya Kalujny]

Hey All,

Due to the problem mentioned here:
http://social.msdn.microsoft.com/Forums/en/vclanguage/thread/191de00a-53c9-4bd9-9cb6-e844eb224ca2
building mysqlpp using MSVC2010 produced a .lib that causes multiply
defined symbols errors when linked to.

I attached a quick & dirty patch that fixes it to this message. Maybe
it should go into repo to save other people some trouble.

Thanks,
Ilya.