Update Gnulib to v0.1-4379-g2ef5a9b4b

Also bump required autoconf version to 2.64, as required by Gnulib.

lib/alignof.h

@@ -1,5 +1,5 @@
 /* Determine alignment of types.
-   Copyright (C) 2003-2004, 2006, 2009-2017 Free Software Foundation, Inc.
+   Copyright (C) 2003-2004, 2006, 2009-2021 Free Software Foundation, Inc.
 
    This program is free software; you can redistribute it and/or modify
    it under the terms of the GNU Lesser General Public License as published by
@@ -12,7 +12,7 @@
    GNU Lesser General Public License for more details.
 
    You should have received a copy of the GNU Lesser General Public License
-   along with this program; if not, see <http://www.gnu.org/licenses/>.  */
+   along with this program; if not, see <https://www.gnu.org/licenses/>.  */
 
 #ifndef _ALIGNOF_H
 #define _ALIGNOF_H
@@ -36,12 +36,14 @@
 /* alignof_type (TYPE)
    Determine the good alignment of an object of the given type at compile time.
    Note that this is not necessarily the same as alignof_slot(type).
-   For example, with GNU C on x86 platforms: alignof_type(double) = 8, but
+   For example, with GNU C on x86 platforms and with clang on Linux/x86:
+   alignof_type(long long) = 8, but alignof_slot(long long) = 4.
+   And alignof_type(double) = 8, but
    - when -malign-double is not specified:  alignof_slot(double) = 4,
    - when -malign-double is specified:      alignof_slot(double) = 8.
    Note: The result cannot be used as a value for an 'enum' constant,
    due to bugs in HP-UX 10.20 cc and AIX 3.2.5 xlc.  */
-#if defined __GNUC__ || defined __IBM__ALIGNOF__
+#if defined __GNUC__ || defined __clang__ || defined __IBM__ALIGNOF__
 # define alignof_type __alignof__
 #else
 # define alignof_type alignof_slot