Add tentative support for FreeMiNT.
* lib/fbufmode.c (fbufmode) [__MINT__]: Add conditional code.
- * lib/fflush.c (clear_ungetc_buffer): Likewise.
* lib/fpurge.c (fpurge): Likewise.
* lib/freadable.c (freadable): Likewise.
* lib/freading.c (freading): Likewise.