* doc/as.texinfo: Updated for -MD option.

    * doc/as.texinfo: Updated for -MD option.
        * Makefile.in (CFILES): Added depend.c.
        (OBJS): Added depend.o.
        * as.h (start_dependencies, register_dependency,
        print_dependencies): New declarations.
        * depend.c: New file.
        * as.c (parse_args): Added -MD option.
        (main): Call print_dependencies.
        (show_usage): Added help for -MD.
        * read.c (s_app_file): Call register_dependency.
        (s_include): Call register_dependency when file is found.
        (read_a_source_file): Call register_dependency.