fbufmode: Fix compilation error on glibc >= 2.28 systems

[Bruno Haible]

Compiling a gnulib testdir on CentOS 8, I get this compilation error:

../../gllib/fbufmode.c: In function 'fbufmode':
../../gllib/fbufmode.c:43:20: error: '_IO_UNBUFFERED' undeclared (first use in 
this function)
   if (fp->_flags & _IO_UNBUFFERED)
                    ^~~~~~~~~~~~~~
../../gllib/fbufmode.c:43:20: note: each undeclared identifier is reported only 
once for each function it appears in
make[4]: *** [Makefile:8935: fbufmode.o] Error 1

This patch fixes it.


diff --git a/lib/stdio-impl.h b/lib/stdio-impl.h
index 4260468..d496257 100644
--- a/lib/stdio-impl.h
+++ b/lib/stdio-impl.h
@@ -18,11 +18,16 @@
    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
+/* Glibc 2.28 made _IO_UNBUFFERED and _IO_IN_BACKUP private.  For now, work
+   around this problem by defining them ourselves.  FIXME: Do not rely on glibc
    internals.  */
-#if !defined _IO_IN_BACKUP && defined _IO_EOF_SEEN
-# define _IO_IN_BACKUP 0x100
+#if defined _IO_EOF_SEEN
+# if !defined _IO_UNBUFFERED
+#  define _IO_UNBUFFERED 0x2
+# endif
+# if !defined _IO_IN_BACKUP
+#  define _IO_IN_BACKUP 0x100
+# endif
 #endif
 
 /* BSD stdio derived implementations.  */