Re-instate the chroot by default code.

Index: configure.ac
--- configure.ac.orig
+++ configure.ac
@@ -6,6 +6,31 @@ AC_CONFIG_HEADER([config.h])
 
 RULE="------------------------------------------------------------"
 
+# Let user specify CHROOT_DIR, or else autodetect it, or else die.
+AC_ARG_WITH(chroot-dir, AS_HELP_STRING([--with-chroot-dir],
+ [specify the chroot directory (default: /var/empty)]),
+ [if test "x$withval" = "xyes"; then
+   AC_MSG_ERROR([please specify --with-chroot-dir=/path/to/darkstat/chroot])
+  fi
+  _chd="$withval"],
+ [# Find an "empty" directory to serve as the chroot.
+  _chd="/var/empty"
+  AC_MSG_CHECKING([for $_chd])
+  if test -d $_chd ; then
+   AC_MSG_RESULT(found it)
+  else
+   AC_MSG_RESULT(not there)
+   _chd="/var/lib/empty"
+   AC_MSG_CHECKING([for $_chd])
+   if test -d $_chd ; then
+    AC_MSG_RESULT(found it)
+   else
+    AC_MSG_RESULT(not there either)
+    AC_MSG_ERROR([please specify --with-chroot-dir=/path/to/darkstat/chroot])
+   fi
+  fi])
+AC_DEFINE_UNQUOTED(CHROOT_DIR, "$_chd", [Default chroot directory.])
+
 # Allow configure-time override of PRIVDROP_USER.
 AC_ARG_WITH(privdrop-user, AS_HELP_STRING([--with-privdrop-user],
  [specify which user to drop privileges to (default: nobody)]),