Test the __fpending function.

This ensures that if there is an error in the definition of the
PENDING_OUTPUT_N_BYTES expression, we'll find about it right away;
that value is used only in the rare event that close_stdout's
fclose fails with EBADF.

lib/t-fpending.c

@@ -0,0 +1,24 @@
+/* Ensure that __fpending works.  */
+
+#include <config.h>
+#include <stdio.h>
+#include <stdlib.h>
+
+#include "__fpending.h"
+
+int
+main ()
+{
+  if (__fpending (stdout) != 0)
+    abort ();
+
+  fputs ("foo", stdout);
+  if (__fpending (stdout) != 3)
+    abort ();
+
+  fflush (stdout);
+  if (__fpending (stdout) != 0)
+    abort ();
+
+  exit (0);
+}