gnu: m4: Fix FTBFS with glibc >= 2.28.

* gnu/packages/patches/m4-gnulib-libio.patch: New file.
* gnu/local.mk (dist_patch_DATA): Register it.
* gnu/packages/m4.scm (m4)[source](patches): New field.

gnu/local.mk

@@ -960,6 +960,7 @@ dist_patch_DATA =						\
   %D%/packages/patches/mupen64plus-video-z64-glew-correct-path.patch    \
   %D%/packages/patches/mutt-store-references.patch		\
   %D%/packages/patches/myrepos-CVE-2018-7032.patch		\
+  %D%/packages/patches/m4-gnulib-libio.patch			\
   %D%/packages/patches/net-tools-bitrot.patch			\
   %D%/packages/patches/netcdf-date-time.patch			\
   %D%/packages/patches/netcdf-tst_h_par.patch			\

gnu/packages/m4.scm

@@ -32,6 +32,7 @@ (define-public m4
             (method url-fetch)
             (uri (string-append "mirror://gnu/m4/m4-"
                                 version ".tar.xz"))
+            (patches (search-patches "m4-gnulib-libio.patch"))
             (sha256
              (base32
               "01sfjd5a4waqw83bibvmn522g69qfqvwig9i2qlgy154l1nfihgj"))))

gnu/packages/patches/m4-gnulib-libio.patch

@@ -0,0 +1,128 @@
+Adjust the bundled gnulib to cope with removal of libio interface in
+glibc 2.28.
+
+Based on this upstream patch, without hunks that do not apply to m4:
+https://git.savannah.gnu.org/cgit/gnulib.git/commit/?id=4af4a4a71827c0bc5e0ec67af23edef4f15cee8e
+
+diff --git a/lib/fflush.c b/lib/fflush.c
+index 983ade0..a6edfa1 100644
+--- a/lib/fflush.c
++++ b/lib/fflush.c
+@@ -33,7 +33,7 @@
+ #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 void
+@@ -72,7 +72,7 @@ clear_ungetc_buffer (FILE *fp)
+ 
+ #endif
+ 
+-#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 (defined __sferror || defined __DragonFly__ || defined __ANDROID__) && defined __SNPT
+ /* FreeBSD, NetBSD, OpenBSD, DragonFly, Mac OS X, Cygwin, Minix 3, Android */
+@@ -148,7 +148,7 @@ rpl_fflush (FILE *stream)
+   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 --git a/lib/fpending.c b/lib/fpending.c
+index c84e3a5..789f50e 100644
+--- a/lib/fpending.c
++++ b/lib/fpending.c
+@@ -32,7 +32,7 @@ __fpending (FILE *fp)
+   /* 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->_IO_write_ptr - fp->_IO_write_base;
+ #elif defined __sferror || defined __DragonFly__ || defined __ANDROID__
+   /* FreeBSD, NetBSD, OpenBSD, DragonFly, Mac OS X, Cygwin, Minix 3, Android */
+diff --git a/lib/fpurge.c b/lib/fpurge.c
+index b1d417c..3aedcc3 100644
+--- a/lib/fpurge.c
++++ b/lib/fpurge.c
+@@ -62,7 +62,7 @@ fpurge (FILE *fp)
+   /* 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 --git a/lib/freadahead.c b/lib/freadahead.c
+index c2ecb5b..23ec76e 100644
+--- a/lib/freadahead.c
++++ b/lib/freadahead.c
+@@ -30,7 +30,7 @@ extern size_t __sreadahead (FILE *);
+ 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 --git a/lib/freading.c b/lib/freading.c
+index 73c28ac..c24d0c8 100644
+--- a/lib/freading.c
++++ b/lib/freading.c
+@@ -31,7 +31,7 @@ freading (FILE *fp)
+   /* 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 --git a/lib/fseeko.c b/lib/fseeko.c
+index 0101ab5..193f4e8 100644
+--- a/lib/fseeko.c
++++ b/lib/fseeko.c
+@@ -47,7 +47,7 @@ fseeko (FILE *fp, off_t offset, int whence)
+ #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)
+@@ -123,7 +123,7 @@ fseeko (FILE *fp, off_t offset, int whence)
+           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;
+       fp->_offset = pos;
+ #elif defined __sferror || defined __DragonFly__ || defined __ANDROID__
+diff --git a/lib/stdio-impl.h b/lib/stdio-impl.h
+index 78d896e..05c5752 100644
+--- a/lib/stdio-impl.h
++++ b/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.  */
+