In Emscripten and wasi builds, and presumably for other cross builds, the build file system and the host file system look different. It's natural to want to install into cross-build/$TARGET/lib and then mount that as /lib in the host file system. wasi.py has to mess around with setting PYTHONPATH because prefix is set to a path from the build machine. I think it would simplify this if we distinguish between
prefix-- the path in the build file system where we want to install the fileshost_prefix-- the path in the host file system where getpath.c will look for the files
And similarly for exec_prefix and host_exec_prefix.