(ITS#5888) MSVC 9.0 build issue

[kurt@OpenLDAP.org]

Full_Name: Kurt Zeilenga
Version: HEAD
OS: Windows Server 2008
URL: ftp://ftp.openldap.org/incoming/
Submission from: (NULL) (2002:4b8d:e980:0:217:f2ff:fed0:27f1)
Submitted by: kurt


The current configure script checks for _vsnprintf and then defines a macro
#define vsnprintf _vsnprintf.
But the system has a vsnprintf.  This causes a clash in <stdio.h> which defines
both vsnprintf and _vsnprintf.

configure.in and include/portable.in need a bit of work to detected this
situation....