Simplify by assuming __fpending.

Simplify by assuming __fpending.

Now that Emacs is using the gnulib fpending module,
there's no need for Emacs to have a separate implementation.
* configure.ac (stdio_ext.h, __fpending): Remove now-duplicate checks.
(PENDING_OUTPUT_COUNT, DISPNEW_NEEDS_STDIO_EXT): Remove.
* admin/CPP-DEFINES (PENDING_OUTPUT_COUNT): Remove.
* src/dispnew.c: Include <fpending.h>, not <stdio_ext.h>.
(update_frame_1): Use __fpending, not PENDING_OUTPUT_COUNT.
Do not assume that __fpending's result fits in int.