(ITS#3681) overlays don't build as modules on Windows

[hyc@OpenLDAP.org]

Full_Name: Howard Chu
Version: 2.3/HEAD
OS: WinXP
URL: ftp://ftp.openldap.org/incoming/
Submission from: (NULL) (24.126.120.178)
Submitted by: hyc


This may affect 2.2 as well, but I'm not going to bother with that.

Currently slapd compiles static backends and dynamic backends in two separate
passes, to accomodate Windows and AIX linking requirements. But it builds
overlays in one pass, even though there may be a mix of static and dynamic
overlays. This needs to be reworked along the same lines as for backends.