autoconf: Test FREAD_READS_DIRECTORIES

autoconf: Test FREAD_READS_DIRECTORIES

Add test for FREAD_READS_DIRECTORIES to detect when fread() reads fopen'ed
directory.

Tested on these platforms:

  AIX 5.3 - FREAD_READS_DIRECTORIES=UnfortunatelyYes
  HP-UX B.11.11 - FREAD_READS_DIRECTORIES=UnfortunatelyYes
  HP-UX B.11.23 - FREAD_READS_DIRECTORIES=UnfortunatelyYes
  Linux 2.6.25-rc4 - FREAD_READS_DIRECTORIES=
  Tru64 V5.1 - FREAD_READS_DIRECTORIES=UnfortunatelyYes
  Windows - FREAD_READS_DIRECTORIES=

Signed-off-by: Michal Rokos <michal.rokos@nextsoft.cz>
Tested-by: Mike Ralphson <mike@abacus.co.uk>
Tested-by: Johannes Schindelin <Johannes.Schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>