We are using autoconf 2.13 for Cactus. This release is by now 15 years old and doesn't support Fortran (nor modern C++ features). It is time to update.
2.69 has been out for a long enough time to be tested enough, and not too unlikely to be present on a developers machine by default. Regular users wouldn't need to deal with it anyway. I think it would be interesting to give 2.69 a try.