Use Gnulib libgmp module

Use Gnulib libgmp module

Instead of doing GMP by hand, use the Gnulib libgmp module.
* .gitignore: Add lib/gmp.h.
* admin/merge-gnulib (GNULIB_MODULES): Add libgmp.
* configure.ac (GMP_LIB, GMP_OBJ): Remove.  Gnulib uses the name
LIB_GMP, so all uses changed.  All uses of GMP_OBJ removed.
(HAVE_GMP): Set this from Gnulib’s variables.
* lib/gnulib.mk.in, m4/gnulib-comp.m4: Regenerate.
* lib/mini-gmp-gnulib.c, lib/mini-gmp.c, lib/mini-gmp.h, m4/libgmp.m4:
New files, copied from Gnulib.
* src/bignum.h, test/data/emacs-module/mod-test.c:
Include gmp.h unconditionally.
* src/mini-gmp-emacs.c, src/mini-gmp.c, src/mini-gmp.h:
Remove.  This moves these files from src to lib, and
updates them to the current GMP version.
* test/Makefile.in (GMP_H): New macro.
($(test_module)): Use it to decide whether to compile
mini-gmp-gnulib.c too.