Reported by Martin Storsjö <martin@martin.st> in
<https://lists.gnu.org/archive/html/bug-gnulib/2020-12/msg00002.html>.
* m4/ungetc.m4 (gl_FUNC_UNGETC_WORKS): On macOS, don't define
FUNC_UNGETC_BROKEN. Instead, set gl_ftello_broken_after_ungetc to yes.
* m4/ftello.m4 (gl_FUNC_FTELLO): Invoke gl_FUNC_UNGETC_WORKS, and
arrange to provide the workaround if ftello is broken after ungetc.
* lib/ftello.c: Include <errno.h>, intprops.h.
(ftello) [FTELLO_BROKEN_AFTER_UNGETC]: Implement from scratch.
* modules/ftello (Files): Add m4/ungetc.m4.
(Depends-on): Add intprops.
* doc/posix-functions/ftello.texi: Mention the macOS bug.