Fix C++ behaviour and comments for ALIGNOF_MAX_ALIGN_T

Author: encukou

Include/pyport.h

@@ -746,13 +746,13 @@ extern char * _getpty(int *, int, mode_t, int);
 #endif
 
 // Make sure we have maximum alignment, even if the current compiler
-// does not support max_align_t.
-// - Autoconf report alignment of unknown types to 0.
-// - We don't have the Py_MAX macro yet.
+// does not support max_align_t. Note that:
+// - Autoconf reports alignment of unknown types to 0.
 // - 'long double' has maximum alignment on *most* platforms,
 //   looks like the best we can do for pre-C11 compilers.
 // - The value is tested, see test_alignof_max_align_t
 #if !defined(ALIGNOF_MAX_ALIGN_T) || ALIGNOF_MAX_ALIGN_T == 0
+#   undef ALIGNOF_MAX_ALIGN_T
 #   define ALIGNOF_MAX_ALIGN_T _Alignof(long double)
 #endif