gmp config + propagating errors at top level compile

   Yes this latter is especially important, AFAICT.  You state that you
   replace a malloc with a tmp-alloc in one of the mpn files.  Does this
   "help" gcl, or is it required to make it function correctly?  Have you
   by any chance had the opportunity to request the gmp authors to

I am afraid that change is necessary at least for the current
implementation in GCL.   Basically it is so we can use the strategy
for garbage collection.    I will try to convince the gmp
authors to make the change.    Also I would like to use a relocatable space
strategy for the bignums, but that will require some more minor changes
to gmp.   I think I have almost made them, but there is at least one
that is illuding me. [ (si::SET-GMP-ALLOCATE-RELOCATABLE t) will switch
to relocatable, but some random bugs will appear if you compute a lot!]

The changes I am making are ones that could be included in the
distribution.  But I think for the present we are safer in having a
copy of the gmp in gcl, especially since some users might not have gmp
on their system, and the number of functions we need from gmp is not
so huge.

I think for something like the debian distribution, your doing the 
486 distribution is a fine.    If someone is doing a lot of computations
involving bignums, he can recompile gcl/maxima himself for his machine.