gh-100540: Remove obsolete '--with-system-ffi' configure option (GH-100544)

It has had no effect on non-macOS platforms for a long time, and has had
the non-obvious effect of invoking `pkg_config` and not setting
`-DUSING_APPLE_OS_LIBFFI` on macOS since GH-22855.

Author: zware

Doc/using/configure.rst

@@ -471,11 +471,6 @@ Libraries options
    Build the :mod:`pyexpat` module using an installed ``expat`` library
    (default is no).
 
-.. cmdoption:: --with-system-ffi
-
-   Build the :mod:`_ctypes` extension module using an installed ``ffi``
-   library, see the :mod:`ctypes` module (default is system-dependent).
-
 .. cmdoption:: --with-system-libmpdec
 
    Build the ``_decimal`` extension module using an installed ``mpdec``

Misc/NEWS.d/next/Build/2022-12-26-15-07-48.gh-issue-100540.l6ToSY.rst

@@ -0,0 +1,8 @@
+Removed the ``--with-system-ffi`` ``configure`` option; ``libffi`` must
+now always be supplied by the system on all non-Windows platforms.  The
+option has had no effect on non-Darwin platforms for several releases, and
+in 3.11 only had the non-obvious effect of invoking ``pkg-config`` to
+find ``libffi`` and never setting ``-DUSING_APPLE_OS_LIBFFI``.  Now on
+Darwin platforms ``configure`` will first check for the OS ``libffi`` and
+then fall back to the same processing as other platforms if it is not
+found.

configure

@@ -3705,36 +3705,22 @@ AS_VAR_IF([with_system_expat], [yes], [
 AC_SUBST([LIBEXPAT_CFLAGS])
 AC_SUBST([LIBEXPAT_INTERNAL])
 
-# Check for use of the system libffi library
-AC_MSG_CHECKING(for --with-system-ffi)
-AC_ARG_WITH(system_ffi,
-            AS_HELP_STRING([--with-system-ffi], [build _ctypes module using an installed ffi library, see Doc/library/ctypes.rst (default is system-dependent)]),,,)
-
-if test "$ac_sys_system" = "Darwin"
-then
-    case "$with_system_ffi" in
-        "")
-            with_system_ffi="no"
-            ;;
-        yes|no)
-            ;;
-        *)
-            AC_MSG_ERROR([--with-system-ffi accepts no arguments])
-            ;;
-    esac
-    AC_MSG_RESULT($with_system_ffi)
-else
-    AC_MSG_RESULT(yes)
-    if test "$with_system_ffi" != ""
-    then
-        AC_MSG_WARN([--with(out)-system-ffi is ignored on this platform])
-    fi
-    with_system_ffi="yes"
-fi
-
 dnl detect libffi
 have_libffi=missing
-AS_VAR_IF([with_system_ffi], [yes], [
+AS_VAR_IF([ac_sys_system], [Darwin], [
+  WITH_SAVE_ENV([
+    CFLAGS="-I${SDKROOT}/usr/include/ffi $CFLAGS"
+    AC_CHECK_HEADER([ffi.h], [
+      AC_CHECK_LIB([ffi], [ffi_call], [
+        dnl use ffi from SDK root
+        have_libffi=yes
+        LIBFFI_CFLAGS="-I${SDKROOT}/usr/include/ffi -DUSING_APPLE_OS_LIBFFI=1"
+        LIBFFI_LIBS="-lffi"
+      ])
+    ])
+  ])
+])
+AS_VAR_IF([have_libffi], [missing], [
   PKG_CHECK_MODULES([LIBFFI], [libffi], [have_libffi=yes], [
     WITH_SAVE_ENV([
       CPPFLAGS="$CPPFLAGS $LIBFFI_CFLAGS"
@@ -3748,20 +3734,6 @@ AS_VAR_IF([with_system_ffi], [yes], [
       ])
     ])
   ])
-], [
-  AS_VAR_IF([ac_sys_system], [Darwin], [
-    WITH_SAVE_ENV([
-      CFLAGS="-I${SDKROOT}/usr/include/ffi $CFLAGS"
-      AC_CHECK_HEADER([ffi.h], [
-        AC_CHECK_LIB([ffi], [ffi_call], [
-          dnl use ffi from SDK root
-          have_libffi=yes
-          LIBFFI_CFLAGS="-I${SDKROOT}/usr/include/ffi -DUSING_APPLE_OS_LIBFFI=1"
-          LIBFFI_LIBS="-lffi"
-        ], [have_libffi=no])
-      ])
-    ])
-  ])
 ])
 
 AS_VAR_IF([have_libffi], [yes], [