Unbreak build when building without GMP support.

Unbreak build when building without GMP support.

Add support for a new preprocessor macro EMACS_MODULE_HAVE_MPZ_T to
emacs-module.h.  If this macro is defined, assume that mpz_t is
already defined and don’t include gmp.h.

Don’t document the new macro for now, as it’s unclear whether we want
to support this in modules outside the Emacs tree.

* src/emacs-module.h.in: Allow user to prevent inclusion of gmp.h.

* src/emacs-module.c: Use mini-gmp if GMP is unavailable.  Don’t
include gmp.h.

* src/lisp.h: Don’t require gmp.h.  It’s not needed for lisp.h.

* test/Makefile.in (GMP_LIB, GMP_OBJ): New variables.
($(test_module)): Use them.

* test/data/emacs-module/mod-test.c: Use mini-gmp if GMP is unavailable.