Looking for a technique to recreate a Make include file

[Chris Nelson]

I hope I haven't painted myself into a corner.  I've gotten what seems
to be most of the way through implementing a Makefile and I can't get
the last bit to work.  I hope someone here can suggest a technique to
do what I'm trying to do.

I have what I'll call "bills of materials" in version controlled files
in a source repository and I build something like:

   make VER=x

I want my Makefile to use $(VER) as a tag to retrieve a BOM from the
repository, generate a dependency file to include in the Makefile,
rescan including that dependency, and then build the product.  More
generally, my Makefile may have several targets -- A, B, C, etc. --
and I can build different versions of each so I might do:

   make A VER=x
   make B VER=y
   make C VER=z

and the dependency file includes information about all three targets.

However, creating the dependency file is somewhat expensive so if I
do:

  make A VER=x
  ...make source (not BOM) changes...
  make A VER=x

I'd really like the Makefile to not regenerate the dependency.  And
just to make things as complicated as possible, I might do:

  make A VER=x
  .. change version x of A's BOM and check it in
  make A VER=x

so I need to regenerate the dependency on the second build.

The check out messes up the timestamps used to regenerate the
dependencies so I think I need a way for the dependency file to depend
not on the BOM but on some indication that the BOM changed.

What I've come to is making the BOM checkout happen in a .PHONY target
(so it always gets checked out) and keeping track of the contents of
the last checkout in a ".sig" file (if the signature file is missing
or the contents are different than the signature of the new file, then
the BOM changed), and having the dependency generation depend on the
signatures).  This is something like the following (though I've typed
this in new, not copied it from my Makefile):

  BOMS = $(addsuffix .bom,$(MAKECMDGOALS)
  SIGS = $(subst .bom,.sig,$(BOMS))

  DEP = include.d
  -include $(DEP)

  $(DEP) : $(SIGS)
          ... recreate dependency

  .PHONY: $(BOMS)

  $(BOMS):
          ...checkout TAG=$(VER) $@
          ...if $(subst .bom,.sig,$@) doesn't exist
          ...  create it
          ...else
          ...  if new signature is different from file contents
          ...    update signature file
          ...  endif
          ...endif

But the dependency generation doesn't get tripped when the signature
changes.  I think it's because because $(SIGS) isn't a target, so make
doesn't notice when the $(BOMS) rule updates a signature.

I tried having $(DEP) depend directly on the BOMs, checking the BOM
out to a temporary directory and only copying it to the working
directory if it changed and that had different problems.

I tried creating a .sig:.bom rule and managing the timestamps of the
checked out BOM with touch but that didn't work.

Any GNU make wisdom would be appreciated.  Thanks.