ITS#4606 IO error in libldap on Windows XP

[hyc@symas.com]

The configure script was already looking for stdlib.h, but only if WINNT 
was defined. I've changed that to use _WIN32 which I think is more 
appropriate.

I changed liblber and libldap to more extensively avoid using errno on 
WIN32, that should address the other issue.

I've committed a number of other MSVC-related patches that have been 
sitting on my laptop for a while as well.
-- 
   -- Howard Chu
   Chief Architect, Symas Corp.  http://www.symas.com
   Director, Highland Sun        http://highlandsun.com/hyc
   OpenLDAP Core Team            http://www.openldap.org/project/