diff -Naur m4-orig/lib/fflush.c m4/lib/fflush.c
--- m4-orig/lib/fflush.c	2020-09-12 14:37:51.600799148 +0200
+++ m4/lib/fflush.c	2020-09-12 14:40:44.292795458 +0200
@@ -31,7 +31,7 @@
 #undef fflush
 
 
-#if defined _IO_ftrylockfile || __GNU_LIBRARY__ == 1 /* GNU libc, BeOS, Haiku, Linux libc5 */
+#if defined _IO_EOF_SEEN || __GNU_LIBRARY__ == 1 /* GNU libc, BeOS, Haiku, Linux libc5 */
 
 /* Clear the stream's ungetc buffer, preserving the value of ftello (fp).  */
 static inline void
@@ -138,7 +138,7 @@
   if (stream == NULL || ! freading (stream))
     return fflush (stream);
 
-#if defined _IO_ftrylockfile || __GNU_LIBRARY__ == 1 /* GNU libc, BeOS, Haiku, Linux libc5 */
+#if defined _IO_EOF_SEEN || __GNU_LIBRARY__ == 1 /* GNU libc, BeOS, Haiku, Linux libc5 */
 
   clear_ungetc_buffer_preserving_position (stream);
 
diff -Naur m4-orig/lib/fpurge.c m4/lib/fpurge.c
--- m4-orig/lib/fpurge.c	2020-09-12 14:37:51.632799148 +0200
+++ m4/lib/fpurge.c	2020-09-12 14:41:36.832794335 +0200
@@ -61,7 +61,7 @@
   /* Most systems provide FILE as a struct and the necessary bitmask in
      <stdio.h>, because they need it for implementing getc() and putc() as
      fast macros.  */
-# if defined _IO_ftrylockfile || __GNU_LIBRARY__ == 1 /* GNU libc, BeOS, Haiku, Linux libc5 */
+# if defined _IO_EOF_SEEN || __GNU_LIBRARY__ == 1 /* GNU libc, BeOS, Haiku, Linux libc5 */
   fp->_IO_read_end = fp->_IO_read_ptr;
   fp->_IO_write_ptr = fp->_IO_write_base;
   /* Avoid memory leak when there is an active ungetc buffer.  */
diff -Naur m4-orig/lib/freadahead.c m4/lib/freadahead.c
--- m4-orig/lib/freadahead.c	2020-09-12 14:37:51.572799149 +0200
+++ m4/lib/freadahead.c	2020-09-12 14:40:20.052795976 +0200
@@ -25,7 +25,7 @@
 size_t
 freadahead (FILE *fp)
 {
-#if defined _IO_ftrylockfile || __GNU_LIBRARY__ == 1 /* GNU libc, BeOS, Haiku, Linux libc5 */
+#if defined _IO_EOF_SEEN || __GNU_LIBRARY__ == 1 /* GNU libc, BeOS, Haiku, Linux libc5 */
   if (fp->_IO_write_ptr > fp->_IO_write_base)
     return 0;
   return (fp->_IO_read_end - fp->_IO_read_ptr)
diff -Naur m4-orig/lib/freading.c m4/lib/freading.c
--- m4-orig/lib/freading.c	2020-09-12 14:37:51.600799148 +0200
+++ m4/lib/freading.c	2020-09-12 14:40:26.496795838 +0200
@@ -31,7 +31,7 @@
   /* Most systems provide FILE as a struct and the necessary bitmask in
      <stdio.h>, because they need it for implementing getc() and putc() as
      fast macros.  */
-#if defined _IO_ftrylockfile || __GNU_LIBRARY__ == 1 /* GNU libc, BeOS, Haiku, Linux libc5 */
+#if defined _IO_EOF_SEEN || __GNU_LIBRARY__ == 1 /* GNU libc, BeOS, Haiku, Linux libc5 */
   return ((fp->_flags & _IO_NO_WRITES) != 0
           || ((fp->_flags & (_IO_NO_READS | _IO_CURRENTLY_PUTTING)) == 0
               && fp->_IO_read_base != NULL));
diff -Naur m4-orig/lib/fseeko.c m4/lib/fseeko.c
--- m4-orig/lib/fseeko.c	2020-09-12 14:37:51.572799149 +0200
+++ m4/lib/fseeko.c	2020-09-12 14:40:32.032795720 +0200
@@ -41,7 +41,7 @@
 #endif
 
   /* These tests are based on fpurge.c.  */
-#if defined _IO_ftrylockfile || __GNU_LIBRARY__ == 1 /* GNU libc, BeOS, Haiku, Linux libc5 */
+#if defined _IO_EOF_SEEN || __GNU_LIBRARY__ == 1 /* GNU libc, BeOS, Haiku, Linux libc5 */
   if (fp->_IO_read_end == fp->_IO_read_ptr
       && fp->_IO_write_ptr == fp->_IO_write_base
       && fp->_IO_save_base == NULL)
@@ -107,7 +107,7 @@
           return -1;
         }
 
-#if defined _IO_ftrylockfile || __GNU_LIBRARY__ == 1 /* GNU libc, BeOS, Haiku, Linux libc5 */
+#if defined _IO_EOF_SEEN || __GNU_LIBRARY__ == 1 /* GNU libc, BeOS, Haiku, Linux libc5 */
       fp->_flags &= ~_IO_EOF_SEEN;
 #elif defined __sferror || defined __DragonFly__ /* FreeBSD, NetBSD, OpenBSD, DragonFly, MacOS X, Cygwin */
 # if defined __CYGWIN__
diff -Naur m4-orig/lib/stdio-impl.h m4/lib/stdio-impl.h
--- m4-orig/lib/stdio-impl.h
+++ m4/lib/stdio-impl.h
@@ -18,6 +18,12 @@
    the same implementation of stdio extension API, except that some fields
    have different naming conventions, or their access requires some casts.  */
 
+/* Glibc 2.28 made _IO_IN_BACKUP private.  For now, work around this
+   problem by defining it ourselves.  FIXME: Do not rely on glibc
+   internals.  */
+#if !defined _IO_IN_BACKUP && defined _IO_EOF_SEEN
+# define _IO_IN_BACKUP 0x100
+#endif
 
 /* BSD stdio derived implementations.  */
 
--- m4-new/Makefile.am	2020-09-12 14:58:09.544773119 +0200
+++ m4/Makefile.am	2020-09-12 14:58:31.104772658 +0200
@@ -20,7 +20,7 @@
 ##
 ## Written by Gary V. Vaughan <gary@gnu.org>
 
-SUBDIRS = . examples lib src doc checks tests
+SUBDIRS = . examples lib src checks tests
 
 syntax_check_exceptions =		\
   .x-sc_prohibit_tab_based_indentation	\
--- m4/Makefile.in	2020-09-12 15:02:23.100767700 +0200
+++ m4-new/Makefile.in	2020-09-12 15:02:13.312767909 +0200
@@ -904,7 +904,7 @@
 top_build_prefix = @top_build_prefix@
 top_builddir = @top_builddir@
 top_srcdir = @top_srcdir@
-SUBDIRS = . examples lib src doc checks tests
+SUBDIRS = . examples lib src checks tests
 syntax_check_exceptions = \
   .x-sc_prohibit_tab_based_indentation	\
   .x-update-copyright