Generate nt/gnulib.mk from lib/gnulib.mk

Generate nt/gnulib.mk from lib/gnulib.mk

This was proposed by Paul Eggert <eggert@cs.ucla.edu>,
with the purpose of avoiding manual maintenance of
nt/gnulib.mk.

* nt/gnulib-modules-to-delete.cfg: New file.
* nt/Makefile.in (AM_V_GEN, am__v_GEN_, am__v_GEN_0)
(am__v_GEN_1): New variables.
(${srcdir}/gnulib.mk): Rules to generate gnulib.mk from
lib/gnulib.mk and list of modules in gnulib-modules-to-delete.cfg.

* make-dist (nt): Add gnulib-modules-to-delete.cfg to the list of
files to link.
* configure.ac (GNULIB_MK): Compute the value according to $opsys.
* autogen.sh: Create nt/gnulib.mk if it doesn't exist, before
running autoreconf.
* Makefile.in (gnulib_mk): New variable.
($(srcdir)/nt/gnulib.mk): Rule to produce it.
(AUTOMAKE_INPUTS): Use $(gnulib_mk) instead of a literal file
name.
* .gitignore: Add nt/gnulib.mk.

* src/w32.c (acl_errno_valid): Implement it here, as we no longer
build the acl-permissions module from Gnulib.