Don't put compat queue.h into DESTDIR so that it doesn't get installed.

Use a separate compat dir instead.  This still need a bit of finetuning,
though, since the compat dir should be consulted only for non-kernel
components.  However, the current CPPFLAGS granularity doesn't allow that,
and needs a bit of redesign-coming-your-way-soon.

Issue #28