gmp.h needs to detect bit width more dynamically to support universal builds on mac

[Torbjorn Granlund]

nisse at lysator.liu.se (Niels Möller) writes:

  It's a known problem, and solving it is not beneficial just for MacOS.
  E.g., it would be nice to be able to install both 32-bit and 64-bit gmp
  in parallel on, e.g., gnu/linux x86_64, with libraries in different
  directories, say /usr/lib and /use/lib32, but sharing the same
  $prefix/include/gmp.h. In this case, it would be nice to automagically
  chose default $libdir depending on the ABI (I try do that in Nettle's
  configure script, see
  http://git.lysator.liu.se/nettle/nettle/blobs/master/configure.ac#line170),
  but it's a bit messy).
  
This is a can of worms.  I think a GMP-specific solution to this problem
would be a mistake.  Instead, a GNU-wide solution is needed.

If each GNU library did it their way, we'd surely make a mess by
creating variations for installation paths.

No operating system (or GNU/Linux distribution) seem to agree on ABI
installation paths and policies.  To make matters worse, some have
changes with release (for good reasons, I am sure).

I GNU-wide solution need either resillient configure tests for paths, or
a database of paths.

Building a library for multiple ABIs is sometimes known as 'multilib'.

Torbjörn