finally actually using gnulib

[John W. Eaton]

I've made some additional changes to start using gnulib in Octave.  A
preliminary changeset is here:

  jweaton.org/gnulib-diffs.gz

The biggest change is that you have to have git to check out the
gnulib sources.  This requirement only applies to building from
Octave's Mercuial archive, not from a distribution or snapshot
tarball.

There is also a new file called HACKING in the archive that is
intended to explain the tools required to build Octave from a Mercuial
checkout.  This file should probably be expanded to also include
information about the library dependencies (blas, lapack, fftw, ...),
the release procedure, etc.  I'll leave that for another day.

For the initial test of using gnulib, I chose to replace
liboctave/strftime.c with the gnulib version of strftime.  Unless
there are seious objections, I will start to replace other portability
and replacement functions gnulib versions soon.

This change is working fine for me, so unless problems are reported, I
intend to check it in within the next few days.

Comments?

jwe