Backport commit from ruby core (#131)

ruby/ruby@12ec1fb

Co-authored-by: 卜部昌平 <shyouhei@ruby-lang.org>

Author: hsbt

ext/fiddle/fiddle.h

@@ -199,7 +199,10 @@
 /* GCC releases before GCC 4.9 had a bug in _Alignof.  See GCC bug 52023
    <https://gcc.gnu.org/bugzilla/show_bug.cgi?id=52023>.
    clang versions < 8.0.0 have the same bug.  */
-#if (!defined(__STDC_VERSION__) || __STDC_VERSION__ < 201112 \
+#if defined(HAVE__ALIGNOF)
+# /* Autoconf detected availability of a sane `_Alignof()`. */
+# define ALIGN_OF(type) RB_GNUC_EXTENSION(_Alignof(type))
+#elif (!defined(__STDC_VERSION__) || __STDC_VERSION__ < 201112 \
      || (defined(__GNUC__) && __GNUC__ < 4 + (__GNUC_MINOR__ < 9) \
          && !defined(__clang__)) \
      || (defined(__clang__) && __clang_major__ < 8))