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/strnlen.m4

@@ -0,0 +1,32 @@
+# strnlen.m4 serial 12
+dnl Copyright (C) 2002-2003, 2005-2007, 2009-2010 Free Software Foundation,
+dnl 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.
+
+AC_DEFUN([gl_FUNC_STRNLEN],
+[
+  AC_REQUIRE([gl_HEADER_STRING_H_DEFAULTS])
+
+  dnl Persuade glibc <string.h> to declare strnlen().
+  AC_REQUIRE([AC_USE_SYSTEM_EXTENSIONS])
+
+  AC_CHECK_DECLS_ONCE([strnlen])
+  if test $ac_cv_have_decl_strnlen = no; then
+    HAVE_DECL_STRNLEN=0
+  else
+    AC_FUNC_STRNLEN
+    dnl Note: AC_FUNC_STRNLEN does AC_LIBOBJ([strnlen]).
+    if test $ac_cv_func_strnlen_working = no; then
+      REPLACE_STRNLEN=1
+    fi
+  fi
+  if test $HAVE_DECL_STRNLEN = 0 || test $REPLACE_STRNLEN = 1; then
+    AC_LIBOBJ([strnlen])
+    gl_PREREQ_STRNLEN
+  fi
+])
+
+# Prerequisites of lib/strnlen.c.
+AC_DEFUN([gl_PREREQ_STRNLEN], [:])