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.