Fix WASI build

[alex-semenyuk]

No description provided.

[emmatyping]

  --- stderr
  /usr/include/features-time64.h:20:10: fatal error: 'bits/wordsize.h' file not found

  thread 'main' (22339) panicked at Modules/cpython-sys/build.rs:61:10:
  Unable to generate bindings: ClangDiagnostic("/usr/include/features-time64.h:20:10: fatal error: 'bits/wordsize.h' file not found\n")
  note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
make: *** [Makefile:3501: cpython-sys] Error 101

Looks like the system headers are not found correctly by bindgen, I wonder if we need to find wasi-libc/wasi-sdk ourselves. Ideally we'd make sure it's the same as the one found in https://github.com/python/cpython/blob/main/Tools/wasm/wasi/__main__.py, perhaps that script can pass an environment variable the build.rs can read?