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).
the normal case when BUILDLINK_DEPENDS.<pkg> isn't specified, it receives
a value only once due to the multiple inclusion protection in the
bulldlink3.mk files. In the case where a package includes several
buildlink3.mk files that each want a slightly different version of another
dependency, having BUILDLINK_DEPENDS.<pkg> be a list allows for the
strictest <pkg> dependency to be matched.
