stdioext: Add support for Minix.

* lib/fbufmode.c (fbufmode) [__minix]: Add conditional code.
* lib/fpurge.c (fpurge): Likewise.
* lib/freadahead.c (freadahead): Likewise.
* lib/freadable.c (freadable): Likewise.
* lib/freading.c (freading): Likewise.
* lib/freadptr.c (freadptr): Likewise.
* lib/freadseek.c (freadptrinc): Likewise.
* lib/fseeko.c (rpl_fseeko): Likewise.
* lib/fseterr.c (fseterr): Likewise.
* lib/fwritable.c (fwritable): Likewise.
* lib/fwriting.c (fwriting): Likewise.
* lib/fflush.c (clear_ungetc_buffer): Update comment.
* m4/fpending.m4 (gl_PREREQ_FPENDING): Add a variant for Minix.