* automake.in (@installdirs): Remove.

* automake.in (@installdirs): Remove.
(&file_contents): Be able to grow Automake macros with `+='.
(&handle_texinfo, &handle_man_pages, &am_install_var): Let your
files define _am_installdirs.
(&handle_installdirs): Remove the code, just output `install.am'.
* install.am: New.
* data.am, header.am, libs.am, lisp.am, ltlib.am, mans.am, progs.am,
* python.am, scripts.am, texinfos.am: Extend $(_am_installdirs).