libgmp: remove dependency on havelib * m4/libgmp.m4 (gl_LIBGMP): If gl_HAVE_MODULE_HAVELIB is not defined, use the more-traditional AC_SEARCH_LIBS approach. This should work better with GNU Emacs configuration, which uses pkg-config instead of a havelib-style approach. * modules/havelib (gl_HAVE_MODULE_HAVELIB): New witness macro. * modules/libgmp (Depends-on): Remove havelib.