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>

config.mak.in

@@ -46,4 +46,5 @@ NO_MKDTEMP=@NO_MKDTEMP@
 NO_ICONV=@NO_ICONV@
 OLD_ICONV=@OLD_ICONV@
 NO_DEFLATE_BOUND=@NO_DEFLATE_BOUND@
+FREAD_READS_DIRECTORIES=@FREAD_READS_DIRECTORIES@
 SNPRINTF_RETURNS_BOGUS=@SNPRINTF_RETURNS_BOGUS@

configure.ac

@@ -327,6 +327,26 @@ else
 fi
 AC_SUBST(NO_C99_FORMAT)
 #
+# Define FREAD_READS_DIRECTORIES if your are on a system which succeeds
+# when attempting to read from an fopen'ed directory.
+AC_CACHE_CHECK([whether system succeeds to read fopen'ed directory],
+ [ac_cv_fread_reads_directories],
+[
+AC_RUN_IFELSE(
+	[AC_LANG_PROGRAM([AC_INCLUDES_DEFAULT],
+		[[char c;
+		FILE *f = fopen(".", "r");
+		return f && fread(&c, 1, 1, f)]])],
+	[ac_cv_fread_reads_directories=no],
+	[ac_cv_fread_reads_directories=yes])
+])
+if test $ac_cv_fread_reads_directories = yes; then
+	FREAD_READS_DIRECTORIES=UnfortunatelyYes
+else
+	FREAD_READS_DIRECTORIES=
+fi
+AC_SUBST(FREAD_READS_DIRECTORIES)
+#
 # Define SNPRINTF_RETURNS_BOGUS if your are on a system which snprintf()
 # or vsnprintf() return -1 instead of number of characters which would
 # have been written to the final string if enough space had been available.