Update Gnulib to v0.1-4379-g2ef5a9b4b

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

lib/mbsinit.c

@@ -1,5 +1,5 @@
 /* Test for initial conversion state.
-   Copyright (C) 2008-2017 Free Software Foundation, Inc.
+   Copyright (C) 2008-2021 Free Software Foundation, Inc.
    Written by Bruno Haible <bruno@clisp.org>, 2008.
 
    This program is free software: you can redistribute it and/or modify
@@ -13,7 +13,7 @@
    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/>.  */
 
 #include <config.h>
 
@@ -22,17 +22,7 @@
 
 #include "verify.h"
 
-#if (defined _WIN32 || defined __WIN32__) && !defined __CYGWIN__
-
-/* On native Windows, 'mbstate_t' is defined as 'int'.  */
-
-int
-mbsinit (const mbstate_t *ps)
-{
-  return ps == NULL || *ps == 0;
-}
-
-#else
+#if GNULIB_defined_mbstate_t
 
 /* Platforms that lack mbsinit() also lack mbrlen(), mbrtowc(), mbsrtowcs()
    and wcrtomb(), wcsrtombs().
@@ -45,6 +35,7 @@ mbsinit (const mbstate_t *ps)
    We define the meaning of mbstate_t as follows:
      - In mb -> wc direction, mbstate_t's first byte contains the number of
        buffered bytes (in the range 0..3), followed by up to 3 buffered bytes.
+       See mbrtowc.c.
      - In wc -> mb direction, mbstate_t contains no information. In other
        words, it is always in the initial state.  */
 
@@ -58,4 +49,22 @@ mbsinit (const mbstate_t *ps)
   return pstate == NULL || pstate[0] == 0;
 }
 
+#else
+
+int
+mbsinit (const mbstate_t *ps)
+{
+# if defined _WIN32 && !defined __CYGWIN__
+  /* Native Windows.  */
+  /* MSVC defines 'mbstate_t' as an 8-byte struct; the first 4 bytes matter.
+     On mingw, 'mbstate_t' is sometimes defined as 'int', sometimes defined as
+     an 8-byte struct, of which the first 4 bytes matter.  */
+  return ps == NULL || *(const unsigned int *)ps == 0;
+# else
+  /* Minix, HP-UX 11.00, Solaris 2.6, Interix, ...  */
+  /* Maybe this definition works, maybe not...  */
+  return ps == NULL || *(const char *)ps == 0;
+# endif
+}
+
 #endif