bootstrap: add bootstrap_post_import_hook

bootstrap: add bootstrap_post_import_hook

Bison does still need something like the gnulib_mk_hook whose
invocation I had to remove along with slurp in commit 767ccd40.
Technically, we could get along without it, but doing so would
have required living with a warning and a mandatory post-bootstrap
automake rerun.
* build-aux/bootstrap (gnulib_mk_hook): Remove definition, too.
(bootstrap_post_import_hook): New function.
Invoke it after gnulib-tool --import and before autoreconf.