ghc-wasm-meta instructions fail on ffi.h on Mac ARM

Summary

Following the instructions in ghc-wasm-meta fails on my Mac M2 machine. It's specific to Mac, as it works fine building in Docker on aarch64 ubuntu. Specifically, the ./configure $CONFIGURE_ARGS step fails with

Cannot find ffi.h for system libffi

Looking at the log, the failure is coming from /Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include/ffi/ffi.h which includes os/availibility.h, which ~/.ghc-wasm/wasi-sdk/bin/clang can't find. Adding /Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include to CPATH (where os/availability.h exists) gives different errors. Setting CPATH to ~/.ghc-wasm/wasi-sdk/share/wasi-sysroot/include (with the newly downloaded ffi.h) also doesn't help:

$ echo '#include <ffi.h>' > foo.c
$ (export CPATH=~/.ghc-wasm/wasi-sdk/share/wasi-sysroot/include; source ~/.ghc-wasm/env; $CC -v foo.c)
ignoring nonexistent directory "/Users/bchinn/.ghc-wasm/wasi-sdk/bin/../share/wasi-sysroot/include/wasm32-wasi"
ignoring duplicate directory "/Users/bchinn/.ghc-wasm/wasi-sdk/share/wasi-sysroot/include"
  as it is a non-system directory that duplicates a system directory
#include "..." search starts here:
#include <...> search starts here:
 /Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include/ffi
 /Users/bchinn/.ghc-wasm/wasi-sdk/lib/clang/15.0.7/include
 /Users/bchinn/.ghc-wasm/wasi-sdk/share/wasi-sysroot/include

I got it working by pointing to some other ffi.h file I have, but I also found a better workaround: removing --with-system-libffi seems to work.

Steps to reproduce

1. Download wasi-sdk artifacts per ghc-wasm-meta steps
2. Download wasm-libffi artifacts per ghc-wasm-meta steps
3. Add ~/.ghc-wasm/env per ghc-wasm-meta steps
4. bash -c 'source ~/.ghc-wasm/env && ./boot && ./configure $CONFIGURE_ARGS'

Expected behavior

The last step should be successful.

Environment

• GHC version used:

Optional:

• Operating System:
• System Architecture: