gh-155813: Stop overwriting LIBFFI_CFLAGS and LIBFFI_LIBS on macOS

[advitrocks9]

On macOS, configure sets LIBFFI_CFLAGS and LIBFFI_LIBS from the SDK before the pkg-config
check, so values passed on the command line are ignored. Doc/using/configure.rst documents both
as overrides. This uses the same preserve-if-set form as the pkg-config fallback and the libuuid
probe.

Configuring with LIBFFI_CFLAGS=-I/nonexistent/include/ffi LIBFFI_LIBS="-L/nonexistent/lib -lffi",
the Makefile currently ends up with the SDK paths:

MODULE__CTYPES_CFLAGS=-fno-strict-overflow -I/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include/ffi -DUSING_APPLE_OS_LIBFFI=1 -DUSING_MALLOC_CLOSURE_DOT_C=1
MODULE__CTYPES_LDFLAGS=-lffi -ldl

and with this change keeps the given values:

MODULE__CTYPES_CFLAGS=-fno-strict-overflow -I/nonexistent/include/ffi -DUSING_MALLOC_CLOSURE_DOT_C=1
MODULE__CTYPES_LDFLAGS=-L/nonexistent/lib -lffi -ldl

With no overrides the output is unchanged. USING_APPLE_OS_LIBFFI drops out when overriding
since it guards Apple-libffi code in _ctypes. iOS sets a different ac_sys_system and is
unaffected. The other documented *_CFLAGS/*_LIBS pairs already preserve user values.

• Issue: macOS: configure does not support linking against a non-system libffi #155813