bootstrap: don't let a user's CDPATH setting affect this script

bootstrap: don't let a user's CDPATH setting affect this script

When CDPATH is set, cd will sometimes generate output.
When "cd" is run in a subshell whose output matters, that
surprising-to-some output can cause malfunction.
Unsetting CDPATH turns off this shell "feature."
* build-aux/bootstrap (CDPATH): Unset.
Reported by Reuben Thomas in:
http://thread.gmane.org/gmane.comp.lib.gnulib.bugs/30435
and inspired by his patch here:
http://thread.gmane.org/gmane.comp.lib.gnulib.bugs/30440