Thirty year old codebase with few developers, built on multiple platforms. It's about what you'd expect. But I believe there have been some improvements made in the past five years or so (the source was released under GPLv3 about ten years ago).

BEST, llc has built it with Tup (see https://github.com/iocane/unbox), so changing the build system is at least possible.

Edit: there's also a long-open PR to move to CMake at https://github.com/jsoftware/jsource/pull/20.