dnl Available from the GNU Autoconf Macro Archive at:
dnl http://www.gnu.org/software/ac-archive/htmldoc/ac_define_dir.html
dnl
AC_DEFUN([AC_DEFINE_DIR], [
  test "x$prefix" = xNONE && prefix="$ac_default_prefix"
  test "x$exec_prefix" = xNONE && exec_prefix='${prefix}'
  ac_define_dir=`eval echo [$]$2`
  ac_define_dir=`eval echo [$]ac_define_dir`
  $1="$ac_define_dir"
  AC_SUBST($1)
  ifelse($3, ,
    AC_DEFINE_UNQUOTED($1, "$ac_define_dir"),
    AC_DEFINE_UNQUOTED($1, "$ac_define_dir", $3))
])