This is a lot changed here. Is all this just NixOS support?
There are only a few lines of real changes to configure.in and make.configuration. The other changes are caused by regenerating configure and updating config.guess to a newer version.
One of the changes sets LD to CXX if LD was set to 'ld'. Before, if LD was set to anything, that setting was honored. If LD is set to 'ld' on NixOS by default, would it be possible to set LD to something else than 'ld' (CXX) in the configuration file, instead of hard-coding this into Cactus make? That way, on all systems (not just NixOS), the setting of LD would be continued to be honored.
Cactus expects LD to be the C++ compiler driver to be used for linking, not the linker. In other words, setting LD to "ld" is almost guaranteed to be wrong, and setting it to the value of CXX is a good default choice. Since the Cactus definition of LD goes against the grain of what LD means everywhere else (e.g. in autotools), handling this problematic case generically is better than applying a NixOS-only solution.
I have resumed the review.
I agree with Frank that I would rather keep LD at whatever value the user set it to (so that it is less surprising) rather than second guess.
I would prefer config.guess’s update to be a separate commit stating just that “update config.guess to XX.YY version” whichever one it may be (assuming you took it from its official repo at http://savannah.gnu.org/projects/config/).