Log Message:
when Apache is built with dlcompat present, the installed httpd/os.h
header file #includes <dlfcn.h>, so we need to include
dlopen.buildlink3.mk so that dlfcn.h can be found by packages which
use httpd/os.h.

XXX this is not entirely correct, but works around the problem
XXX sufficiently. the problem is that Darwin (7.7.x) has dlopen() and
XXX friends but does not provide prototypes in dlfcn.h (or anywhere else).