build: not cross-compiling when musl from gnu

[davidhewitt]

Fixes #1264

This patch allows compiling for musl on linux (and possibly a few other cases, I tried to write a general patch which was reasonable).

I think as long as the extension module is statically linked against libc (e.g. such as in the musl case) then there is no issue loading such an extension module. I was able to test locally that building a musl extension module worked fine.

[kngwyu]

So we cannot use PYO3_CROSS_LIB_DIR in this setting?
I'm not familiar with cross-compiling, but isn't this hurt some generality?
E.g., we use PY_MAJOR_VERSION and Py_ENABLE_SHARED from PYO3_CROSS_LIB_DIR.

[davidhewitt]

We don't need PYO3_CROSS_LIB_DIR in this setting at all - we can use the normal PYO3_PYTHON (or PATH) in this case and extract the build configuration using get_config_from_interpreter.

The extension module statically linked with musl libc can be run with the normal system python - so it's not really a cross-compile. Just linking against a different libc.

[kngwyu]

Ah, that makes sense, thanks.