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.