The idea is to let programs simply include <gmp.h>, and
so long as they live within the mini-gmp subset they need
not worry about whether the GMP libraries are installed.
* MODULES.html.sh: Mention it.
* config/srclist.txt: Mention files copied from GMP source.
* config/srclistvars.sh (GMP): New var.
* lib/mini-gmp-gnulib.c, m4/libgmp.m4, modules/libgmp:
* modules/libgmp-tests, tests/test-libgmp.c: New files.
* lib/mini-gmp.c, lib/mini-gmp.h: New files, copied from GMP.