Add support to getopt for Emacs, which doesn't use LIBOBJS in the

Add support to getopt for Emacs, which doesn't use LIBOBJS in the
normal way.
* getopt.m4 (gl_GETOPT_SUBSTITUTE_HEADER): New macro.
(gl_GETOPT_SUBSTITUTE): Use it.  Invoke gl_PREREQ_GETOPT.
(gl_GETOPT_IFELSE, gl_GETOPT_CHECK_HEADERS): New macros.
(gl_GETOPT): Use the new macros.  Most of the implementation
is moved to the new macros.  This is for programs like Emacs
that don't want all the functionality of gl_GETOPT.