botan-src does not respect cargo's target triple when invoking configure.py

[olvrlrnz]

I'm trying to build a binary on my Apple Silicon mac mini which depends on botan-rs with enabled features vendored and botan3. Since we want to create a universal binary with lipo, we have to build targets aarch64-apple-darwin and x86_64-apple-darwin.

When botan-src builds the C library, it calls configure.py but the target initially passed to cargo is completely ignored.
Although there is a list of options that can be overridden with BOTAN_CONFIGURE_ env vars, I think --os and --cpu should be set depending on cargo --target and not via BOTAN_CONFIGURE_CPU and BOTAN_CONFIGURE_OS. I cannot think of any situation where setting CPU and OS to something other than what has been passed in --target would make sense.

Dumping the environment in pub fn build() -> (String, String) shows

CARGO_CFG_TARGET_ARCH=aarch64
CARGO_CFG_TARGET_ENDIAN=little
CARGO_CFG_TARGET_ENV=
CARGO_CFG_TARGET_FAMILY=unix
CARGO_CFG_TARGET_OS=macos
CARGO_CFG_TARGET_POINTER_WIDTH=64
CARGO_CFG_TARGET_VENDOR=apple
CARGO_CFG_UNIX=
...
TARGET=aarch64-apple-darwin

So the build script definitely knows what should be built, but doesn't pass the information to configure.py.

• What I did:

$ cargo build -p botan --features vendored,botan3 --target x86_64-apple-darwin

• What I expected:

The library is built for x86_64

• What actually happened:

The library was built for aarch64 according to ld and the log file

   INFO: configure.py invoked with options "--with-build-dir=/Users/username/src/work/botan-rs/target/debug/build/botan-sys-b2a7d959a11e9080/out/botan --build-targets=static --without-documentation --no-install-python-module --distribution-info=https://crates.io/crates/botan-src --with-debug-info"
   INFO: Configuring to build Botan 3.5.0 (revision git:2e5c7d3eb0dcb63d087fc3e7c92010aa82509bbf)
   INFO: Python version: "3.12.5 (main, Aug  6 2024, 19:08:49) [Clang 15.0.0 (clang-1500.3.9.4)]"
   INFO: Autodetected platform information: OS="Darwin" machine="arm64" proc="arm"
   INFO: Guessing target OS is darwin (use --os to set)
   INFO: Guessing to use compiler xcode (use --cc or CXX to set)
   INFO: Guessing target processor is a arm64 (use --cpu to set)
   INFO: Using /etc/ssl/cert.pem as system certificate store
   INFO: Auto-detected compiler version xcode 15.3
   INFO: Auto-detected compiler arch arm64
   INFO: Target is xcode:15.3-macos-arm64
   INFO: Assuming target arm64 is little endian