Use gnulib-tool to create gnulib source files.

* Add gnulib files generated by gnulib-tool in build-aux, m4 and
    grub-core/gnulib directories
    * .bzignore: Add **/.deps and autogenerated gnulib files
    * configure.ac: Assign auxiliary directory to build-aux, add invocation
    of gnulib macros, add grub-core/gnulib/Makefile
    * Makefile.am: Add gnulib directory in SUBDIRS (removing unnecessary .),
    include m4 directory to aclocal.
    * Makefile.util.def: Remove direct compilation of gnulib source files
    and use the new grub-core/gnulib/libgnu.a.
    * build-aux/config.rpath: move config.rpath from top directory to
    build-aux
    * conf/Makefile.common: Remove the macro _GL_UNUSED already defined
    in gnulib headers
    * conf/Makefile.extra-dist: Add m4/gnulib-cache.m4
    * grub-core/Makefile.core.def: Remove unnecessary extra_dist
    * grub-core/lib/posix_wrap/localcharset.h (locale_charset): Update
    header.
    * grub-core/lib/posix_wrap/langinfo.h (nl_langinfo): Return static
    string.

m4/ssize_t.m4

@@ -0,0 +1,23 @@
+# ssize_t.m4 serial 5 (gettext-0.18.2)
+dnl Copyright (C) 2001-2003, 2006, 2010 Free Software Foundation, Inc.
+dnl This file is free software; the Free Software Foundation
+dnl gives unlimited permission to copy and/or distribute it,
+dnl with or without modifications, as long as this notice is preserved.
+
+dnl From Bruno Haible.
+dnl Test whether ssize_t is defined.
+
+AC_DEFUN([gt_TYPE_SSIZE_T],
+[
+  AC_CACHE_CHECK([for ssize_t], [gt_cv_ssize_t],
+    [AC_COMPILE_IFELSE(
+       [AC_LANG_PROGRAM(
+          [[#include <sys/types.h>]],
+          [[int x = sizeof (ssize_t *) + sizeof (ssize_t);
+            return !x;]])],
+       [gt_cv_ssize_t=yes], [gt_cv_ssize_t=no])])
+  if test $gt_cv_ssize_t = no; then
+    AC_DEFINE([ssize_t], [int],
+              [Define as a signed type of the same size as size_t.])
+  fi
+])