configure.in: refine __builtin_unreachable check

* configure.in (rb_cv_func___builtin_unreachable): try with an
  external variable not only by a warning, which might not be
  shown due to the optimization.  [ruby-core:61647] [Bug #9665]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@45716 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

ChangeLog

@@ -1,3 +1,9 @@
+Fri Apr 25 14:55:59 2014  Nobuyoshi Nakada  <nobu@ruby-lang.org>
+
+	* configure.in (rb_cv_func___builtin_unreachable): try with an
+	  external variable not only by a warning, which might not be
+	  shown due to the optimization.  [ruby-core:61647] [Bug #9665]
+
 Fri Apr 25 13:11:49 2014  NARUSE, Yui  <naruse@ruby-lang.org>
 
 	* configure.in: NetBSD's ksh, used by configure, needs escapes.

configure.in

@@ -1595,8 +1595,8 @@ if test "$GCC" = yes; then
 
     AC_CACHE_CHECK(for __builtin_unreachable, rb_cv_func___builtin_unreachable,
     [RUBY_WERROR_FLAG(
-    [AC_TRY_LINK([@%:@include <stdlib.h>],
-	[exit(0); __builtin_unreachable();],
+    [AC_TRY_LINK([volatile int zero;],
+	[if (zero) __builtin_unreachable();],
 	[rb_cv_func___builtin_unreachable=yes],
 	[rb_cv_func___builtin_unreachable=no])
     ])