Provide a way for downstream crates to access the build configuration

[davidhewitt]

In examples/pyo3-pytests we need to detect the Python version and PyPy so that some tests can be conditionally compiled. We do this by invoking the Python interpreter:

pyo3/examples/pyo3-pytests/build.rs

Lines 1 to 28 in 990bbb5

	use std::process::Command;
	fn main() {
	let out = Command::new("python")
	.args(&["-c", "import sys; import platform; print(sys.version_info[1]); print(platform.python_implementation())"])
	.output()
	.expect("python version did not print");
	let output = String::from_utf8_lossy(&out.stdout);
	let mut lines = output.trim().lines();
	println!("{}", output);
	let version: u8 = lines
	.next()
	.unwrap()
	.parse()
	.expect("python version was not parsed");
	let implementation = lines.next().unwrap();
	for each in 6..version {
	println!("cargo:rustc-cfg=Py_3_{}", each);
	}
	if implementation == "PyPy" {
	println!("cargo:rustc-cfg=PyPy");
	}
	}

This has some downsides that we should try to fix:

• It's using different logic to detect Python compared to PyO3's main build.rs, which could lead to misconfiguration in certain situations. (e.g. it doesn't check PYO3_PYTHON, nor will it work when cross-compiling.)
• Users have to copy this pattern by hand into every crate which needs to support multiple Python versions.

e.g. I've seen a similar trick in the wild in orjson (which is where I borrowed this from) https://github.com/ijl/orjson/blob/master/build.rs

I think we can solve this by splitting out a lot of PyO3's build.rs into a new crate pyo3-build-config which we would use in [build-dependencies]. Downstream crates like pyo3-pytests and orjson could then use this crate also in [build-dependencies] to access the exact same configuration which PyO3 has loaded in their own build.rs files.

I've started playing around with something of this form; this issue is a reminder for me to finish it off.

[davidhewitt]

cc @m-ou-se I wonder if this would "resolve" m-ou-se/inline-python#27 by making it easier for inline-python to request linking to libpython in its build script. (I'm thinking with this change, inline-python's build script could access the pyo3 build config.)

[davidhewitt]

Resolved in #1622