bootstrap: add hook for altering gnulib.mk, for Bison

bootstrap: add hook for altering gnulib.mk, for Bison

* build-aux/bootstrap (gnulib_mk_hook): New function, so that
the Bison bootstrapping process can rewrite file names and variables
in this file before later parts of 'bootstrap' use the file.
Bison wants to include lib/gnulib.mk from the top-level makefile,
so it needs the file names in this file to be relative to the top
level, not relative to lib; plus it needs variable names to be
rewritten.
(slurp): Use the new function.