all: avoid duplicated write errors on FreeBSD

* src/system.h (write_error): Also call fpurge(), which was seen to
be needed on FreeBSD 13.1 to avoid duplicated write errors.
* src/head.c (xwrite_stdout): Likewise.
* bootstrap.conf: Depend on fpurge.
Reported by Bruno Haible.

bootstrap.conf

@@ -104,6 +104,7 @@ gnulib_modules="
   fnmatch-gnu
   fopen-safer
   fprintftime
+  fpurge
   free-posix
   freopen
   freopen-safer

src/head.c

@@ -181,6 +181,7 @@ xwrite_stdout (char const *buffer, size_t n_bytes)
   if (n_bytes > 0 && fwrite (buffer, 1, n_bytes, stdout) < n_bytes)
     {
       clearerr (stdout); /* To avoid redundant close_stdout diagnostic.  */
+      fpurge (stdout);
       error (EXIT_FAILURE, errno, _("error writing %s"),
              quoteaf ("standard output"));
     }

src/system.h

@@ -769,8 +769,9 @@ static inline void
 write_error (void)
 {
   int saved_errno = errno;
-  fflush (stdout);    /* Ensure nothing buffered that might induce an error. */
-  clearerr (stdout);  /* To avoid extraneous diagnostic from close_stdout.  */
+  fflush (stdout);    /* Last attempt to write any buffered data.  */
+  fpurge (stdout);    /* Ensure nothing buffered that might induce an error. */
+  clearerr (stdout);  /* Avoid extraneous diagnostic from close_stdout.  */
   error (EXIT_FAILURE, saved_errno, _("write error"));
 }