Make build system compatible with autotools 2.71.

The autotools 2.7 suite is starting to be used (Ubuntu and Fedora) and has many incompatible changes for our build system. This MR attempts to resolve those and remain compatible with the earlier and more widely used 2.69.

Works for me on Ubuntu 20.02/2.71 and with Centos/2.69 and Ubuntu/2.69.

Rather than worry about 2.7, it would seem a good idea to make sure 2.69 is still working for all the configure combinations we can. Access to less obvious OSes would also be useful tests.

Also closes #536 (closed).