It's basically impossible to program with no side effects in any general purpose language, and providing a build system language likely requires side-effect ric

It's basically impossible to program with no side effects in any general purpose language, and providing a build system language likely requires side-effect rich APIs too (to write files, call external tools, etc). Which is to say that you can perfectly well implement a "clean" build setup with make, and provide an "interface" for parts of the system to tie into. Files and directories are as good an input/output interface as any other, and you can do some simple safeguards with permissions on directories and files. To create a local, intermediate file with make, you create a file in TMPDIR (eg with mktemp or tempfile) and remove it at the end, setting the permissions the way you want for any nested steps not to be able to access it.