I have an application with Makefiles that want to set DESTDIR, to
prefix the target-tree for `install' targets, so that the result of
various different builds get installed in various places.

The extant Makefiles are from an older derivative of bmake, wherein
setting DESTDIR has just the effect I describe.

I haven't looked hard, but the simplest and cleanest way I can see to
get the effect I desire is smoething liek this:

1. Move all the ${DESTDIR:D ...} checks from our
   bsd.{dep,lib,prog,sys}.mk into a new file, say bsd.cross.mk.
   bsd.cross.mk will be  responsible for giving appropriate values
   to  a few new make symbols, say DESTCFLAGS, DESTCXXFLAGS,
   DESTLINTFLAGS, DESTSYSFLAGS.
   

2. Instead of using ${DESTDIR:D} macros directly in in-place rules,
   or setting CFLAGS or CXXFLAGS directly to ${DESTDIR:D ...} expansions,
   bsd.*.mk files will include bsd.cross.mk, and then add DESTCFLAGS
   to CFLAGS, DESTCXXFLAGS to CFLAGS, etc., as appropriate.

3. bsd.cross.mk will have its assignments of the DEST*FLAGS symbols
   all protected by some suitable .ifndef/.endif test, enabling
   those who wish to set DESTDIR but to also disable the NetBSD-normal
   implicit side effects of setting DESTDIR.

I'll be testing on a patch based on this scheme later today.  But is
there some simpler way to achieve this that I'm totally missing?