getopt, argp: Respect rules for use of AC_LIBOBJ. * m4/argp.m4 (gl_ARGP): Don't invoke gl_REPLACE_GETOPT. (gl_REPLACE_GETOPT_ALWAYS): New macro. * m4/getopt.m4 (gl_FUNC_GETOPT_POSIX): Test whether gl_REPLACE_GETOPT_ALWAYS is defined. Set REPLACE_GETOPT.