Improve robustness of 'make bootstrap'.

Improve robustness of 'make bootstrap'.

Run autogen.sh after bootstrap-clean, to avoid bzr pull issues.
* INSTALL, README: Document autogen.sh.
* Makefile.in (Makefile): Mark it as precious, since it's updated
atomically.
(MAKE_CONFIG_STATUS): New macro.
(config.status, bootstrap): Use it.  This causes 'make bootstrap'
to run config.status with the --recheck option, which is more
appropriate for a bootstrap.
(bootstrap): Run autogen.sh right after cleaning.  Don't worry
about failures due to missing tools.
* autogen.sh: Exit with status 101 when failing due to missing tools.
* make-dist: Distribute autogen.sh.