default gnulib_path in build-aux/bootstrap script

[John W. Eaton]

I updated Octave's copy of the bootstrap script from gnulib's version
today and hit an error because gnulib_path was set to ''.  The code in
the bootstrap script for this is

  git_modules_config () {
    test -f .gitmodules && git config --file .gitmodules "$@"
  }

  gnulib_path=`git_modules_config submodule.gnulib.path`
  : ${gnulib_path=gnulib}

I don't have a .gitmodules file, so git_modules_config doesn't return
anything and gnulib_path is set to '' before the

  : ${gnulib_path=gnulib}

line, so then it remains ''.  Shouldn't this be

  : ${gnulib_path:=gnulib}

(or equivalent if this method is not sufficiently portable) so that it
will be set to the default value if $gnulib_path is unset or empty,
not just if it is unset?  It looks to me that $gnulib_path will always
be set, to the default value of "gnulib" will never be used.

jwe