Add support for enabling/disabling installation of specified programs.

Add support for enabling/disabling installation of specified programs.
* NEWS: Mention new configure-time options.
Mention that neither arch nor su is built/installed, by default.
* m4/include-exclude-prog.m4: New file.
* configure.ac: Use new macro, gl_ADD_PROG, rather than
manually appending to OPTIONAL_BIN_PROGS and MAN.
Move the code that adds "df" to the list of programs to build from
m4/jm-macros into this file.
Use gl_INCLUDE_EXCLUDE_PROG, then handle special cases: ginstall, [.
(NO_INSTALL_PROGS_DEFAULT): AC_SUBST it.  Used by man/Makefile.am.
* man/Makefile.am (dist_man_MANS): Remove from this list all man pages
corresponding to "bin" programs. Add $(MAN) instead.
(optional_mans): Remove all uses.
(check-x-vs-1): Adapt to work even though arch and su are typically
no longer built (and neither are their .1 files).
* src/Makefile.am (install_su): Rename from INSTALL_SU, now that
INSTALL_SU has a different meaning. Use the new $(INSTALL_SU) value.