macOS: configure does not support linking against a non-system libffi

Bug report

Bug description:

On macOS, the configure script does not provide a way to link _ctypes against a non-system libffi.

Although LIBFFI_CFLAGS and LIBFFI_LIBS are documented as configuration variables for libffi, configure first checks for the system libffi from Apple's SDK and if found overwrites these variables.
The pkg-config based check, which does preserve these variables, is never run when the system libffi is detected.

The confusing --with-system-ffi option, removed in Python 3.12 (#100544), provided a way to invoke the pkg-config based configuration on macOS, bypassing the detection of the SDK libffi.

I ran into this while updating python-build-standalone to use the standard extension configuration in 3.12+ (astral-sh/python-build-standalone#1180). Other than patching configure the best option seemed to be to use ac_cv_lib_ffi_ffi_call=no to force the check to fail.

I also found that conda-forge patches configure to get around this issue when building CPython.

Related issues

• https://github.com/python/cpython/issues/58732
• https://github.com/python/cpython/pull/100544

CPython versions tested on:

3.12, 3.13, 3.14, 3.15, CPython main branch

Operating systems tested on:

macOS

Linked PRs

• gh-156486

Start with the linked configure.ac checks around lines 4363-4392 and compare them with the generated configure script. Trace how LIBFFI_CFLAGS, LIBFFI_LIBS, the macOS SDK libffi check, and the pkg-config check interact. Done means a macOS build can preserve user-supplied settings and link _ctypes against a non-system libffi.