Cross builds: distinguish between installation prefix and host prefix in configure.ac?

[hoodmane]

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 files
• host_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.

cc @brettcannon @freakboy3742

Linked PRs

• gh-124932: Distinguish build prefix from host prefix in cross builds #124933

[brettcannon]

In regards to the wasi.py reference:

cpython/Tools/wasm/wasi.py

Lines 222 to 228 in 1f9025a

	# Use PYTHONPATH to include sysconfig data which must be anchored to the
	# WASI guest's `/` directory.
	args = {"GUEST_DIR": "/",
	"HOST_DIR": CHECKOUT,
	"ENV_VAR_NAME": "PYTHONPATH",
	"ENV_VAR_VALUE": f"/{sysconfig_data}",
	"PYTHON_WASM": working_dir / "python.wasm"}

[brettcannon]

This has been deemed Emscripten-specific over in the PR.