bug#28793: 26.0.90; Compile error with Emacs 26 on macOS 10.13

[Eli Zaretskii]

> From: George Plymale II <georgie@southernohio.net>
> Date: Thu, 12 Oct 2017 02:19:19 -0400
> 
> open.c:75:27: error: unknown type name 'PROMOTED_MODE_T'
>       mode = va_arg (arg, PROMOTED_MODE_T);
>                           ^
> 1 error generated.
> make[1]: *** [open.o] Error 1

PROMOTED_MODE_T is defined in src/config.h, please see what happened
there.  On my system, I see this in config.h:

  /* Define to the type that is the result of default argument promotions of
     type mode_t. */
  #define PROMOTED_MODE_T int

On another system I see this:

  /* Define to the type that is the result of default argument promotions of
     type mode_t. */
  #define PROMOTED_MODE_T mode_t