Support bootstrap --gnulib-srcdir=DIR.
I saw `bootstrap' doesn't accept --gnulib-srcdir as it is already done
by other projects using gnulib. It doesn't add a new feature as it can
be already done specifying GNULIB_SRCDIR, it just make the script more
similar to scripts used by other projects doing a bootstrap from
gnulib.
* bootstrap: Add --gnulib-srcdir option, which overrides
GNULIB_SRCDIR, for consistency with gnulib bootstrap.
Signed-off-by: Eric Blake <eblake@redhat.com>
(cherry picked from commit
d7879f327c28e6a206aa93d009e27224cf6b4b16)