+#AC_CACHE_VAL(cl_cv_header_darts_version, [
+# AC_TRY_CPP([],cl_cv_header_darts_version=yes,[AC_MSG_ERROR(an old version of darts.h is found. You need to get darts-0.32 or higher from http://chasen.org/~taku/software/darts/.)])])
+#AC_MSG_RESULT(checking for darts.h version >= 0.32... $cl_cv_header_darts_version)