build: update gnulib to latest, and bootstrap
build: use more portable shell syntax in search of working shell

* m4/shell.m4: Adjust sh/case syntax not to evoke a syntax
error from Solaris 10's /bin/sh.
build: avoid warning about deprecated use of automake's ACLOCAL_AMFLAGS

* Makefile.am (ACLOCAL_AMFLAGS): Don't use this deprecated variable.
* configure.ac: Do this instead: AC_CONFIG_MACRO_DIR([m4]).
build: avoid automake warning that suggests use of subdir-objects

* configure.ac (AM_INIT_AUTOMAKE): Use the subdir-objects option.
version 1.6

* NEWS: Record release date.
maint: post-release administrivia

* NEWS: Add header line for next release.
* .prev-version: Record previous version.
* cfg.mk (old_NEWS_hash): Auto-update.