NetBSD make: determining a relative OBJDIR?

[Joseph Koshy <jkoshy%netbsd.org@localhost>]

Consider a source tree with sub-directories (A) and (B),
where the code in directory (B) uses a file that is generated
by the build steps in directory (A).

For an in-place build, the directory hierarchy might look like:

  $(TOP)/
    |
    +-- A/
    |   |
    |   +- generated-file
    |
    +-- B/
    |   |
    |   +- uses-generated-file
    etc.

In the event an 'obj' directory is used, the tree might be
something like:

  $(TOP)/
    |
    +-- A/
    |   |
    |   +- obj/
    |       |
    |       +- generated-file
    |
    +-- B/
    |   |
    |   +- uses-generated-file
    etc.

If MAKEOBJDIRPREFIX is used, the obj/ tree could reside
somewhere else entirely:

  $(TOP)/
    |
    +-- A/
    |
    +-- B/
    |   |
    |   +- uses-generated-file
    etc.

  $(MAKEOBJDIRPREFIX)/$(TOP)/
    |
    +-- A/
    |   |
    |   +- generated-file
    |
    +-- B/
    |
    etc.

What would be the cleanest way to is there a way to determine
the path to an obj/ directory, given relative locations of
source directories (A) and (B)?

For the Elftoolchain build I currently use nested conditional
blocks:

  .if exists(${.OBJDIR}/../A)
     # For in-place builds, or when MAKEOBJDIRPREFIX is used.
  .elif exists(${.CURDIR}/../A/${.OBJDIR:S,${.CURDIR}/,,}
     # For builds using an obj/ directory.
  .endif

I was wondering if there was a better way.

Thanks,
Joseph Koshy