update to latest gnulib

[Jim Meyering]

I've just updated the gnulib submodule to the latest, which required
some minor adjustments:

build: update gnulib to latest
* gnulib: Update submodule to latest.
* bootstrap.conf (gnulib_modules): Add explicit dependency on
dirname-lgpl. Before, we pulled this in via a dependency.
* bootstrap: Update from gnulib.

build: require autoconf-2.64
* configure.ac: Require autoconf-2.64, up from 2.63, to align with gnulib.