Requiring git 1.6.4, just for the optional GNULIB_SRCDIR support,
was too harsh.  Resynchronize from gnulib.

* .gnulib: Import from latest gnulib.
* bootstrap: Re-synchronize from .gnulib/build-aux.
* bootstrap.conf: Drop git to 1.5.5.
* README-hacking: Document use of GNULIB_SRCDIR.