bpo-46670: Test if a macro is defined, not its value (GH-31178)

* audioop.c: #ifdef WORDS_BIGENDIAN
* ctypes.h: #ifdef USING_MALLOC_CLOSURE_DOT_C
* _ctypes/malloc_closure.c: #ifdef HAVE_FFI_CLOSURE_ALLOC
  and #ifdef USING_APPLE_OS_LIBFFI
* pytime.c: #ifdef __APPLE__
* unicodeobject.c: #ifdef HAVE_NON_UNICODE_WCHAR_T_REPRESENTATION