configure.ac: loosen FREAD_READS_DIRECTORIES test program

configure.ac: loosen FREAD_READS_DIRECTORIES test program

We added an FREAD_READS_DIRECTORIES Makefile knob long ago
in cba22528f (Add compat/fopen.c which returns NULL on
attempt to open directory, 2008-02-08) to handle systems
where reading from a directory returned garbage. This works
by catching the problem at the fopen() stage and returning
NULL.

More recently, we found that there is a class of systems
(including Linux) where fopen() succeeds but fread() fails.
Since the solution is the same (having fopen return NULL),
they use the same Makefile knob as of e2d90fd1c
(config.mak.uname: set FREAD_READS_DIRECTORIES for Linux and
FreeBSD, 2017-05-03).

This works fine except for one thing: the autoconf test in
configure.ac to set FREAD_READS_DIRECTORIES actually checks
whether fread succeeds. Which means that on Linux systems,
the knob isn't set (and we even override the config.mak.uname
default). t1308 catches the failure.

We can fix this by tweaking the autoconf test to cover both
cases. In theory we might care about the distinction between
the traditional "fread reads directories" case and the new
"fopen opens directories". But since our solution catches
the problem at the fopen stage either way, we don't actually
need to know the difference. The "fopen" case is a superset.

This does mean the FREAD_READS_DIRECTORIES name is slightly
misleading. Probably FOPEN_OPENS_DIRECTORIES would be more
accurate. But it would be disruptive to simply change the
name (people's existing build configs would fail), and it's
not worth the complexity of handling both. Let's just add a
comment in the knob description.

Reported-by: Øyvind A. Holm <sunny@sunbase.org>
Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>