[3.15] gh-155813: Stop overwriting LIBFFI_CFLAGS and LIBFFI_LIBS on macOS (GH-156486) (#160129)

Ensures that a user-specified LIBFFI_CFLAGS and LIBFFI_LIBS is honored when
building on macOS.
(cherry picked from commit 6839f8c)

Co-authored-by: Advit Arora <advitarora2@gmail.com>

Author: miss-islington

Misc/NEWS.d/next/Build/2026-08-27-23-24-29.gh-issue-155813.LxRQu9.rst

@@ -0,0 +1,3 @@
+On macOS, :option:`LIBFFI_CFLAGS` and :option:`LIBFFI_LIBS` are no longer
+ignored when the SDK provides ``libffi``, so :mod:`ctypes` can again be built
+against a different ``libffi``. Patch by Advit Arora.

configure

@@ -4286,8 +4286,8 @@ AS_VAR_IF([ac_sys_system], [Darwin], [
       AC_CHECK_LIB([ffi], [ffi_call], [
         dnl use ffi from SDK root
         have_libffi=yes
-        LIBFFI_CFLAGS="-I${SDKROOT}/usr/include/ffi -DUSING_APPLE_OS_LIBFFI=1"
-        LIBFFI_LIBS="-lffi"
+        LIBFFI_CFLAGS=${LIBFFI_CFLAGS-"-I${SDKROOT}/usr/include/ffi -DUSING_APPLE_OS_LIBFFI=1"}
+        LIBFFI_LIBS=${LIBFFI_LIBS-"-lffi"}
       ])
     ])
   ])