Add support for __unreachable_return on clang and GCC.

See https://svn.boost.org/trac/boost/ticket/11661.

include/boost/config/compiler/clang.hpp

@@ -255,6 +255,11 @@
 // All versions with __cplusplus above this value seem to support this:
 #  define BOOST_NO_CXX14_DIGIT_SEPARATORS
 #endif
+//
+// __builtin_unreachable:
+#if defined(__has_builtin) && __has_builtin(__builtin_unreachable)
+#define BOOST_UNREACHABLE_RETURN(x) __builtin_unreachable();
+#endif
 
 
 // Unused attribute:

include/boost/config/compiler/gcc.hpp

@@ -278,6 +278,11 @@
 #if __GNUC__ >= 4
 #  define BOOST_ATTRIBUTE_UNUSED __attribute__((unused))
 #endif
+//
+// __builtin_unreachable:
+#if BOOST_GCC_VERSION >= 40800
+#define BOOST_UNREACHABLE_RETURN(x) __builtin_unreachable();
+#endif
 
 #ifndef BOOST_COMPILER
 #  define BOOST_COMPILER "GNU C++ version " __VERSION__

include/boost/config/suffix.hpp

@@ -444,10 +444,12 @@ namespace std {
 // is defined, in which case it evaluates to return x; Use when you have a return
 // statement that can never be reached.
 
-#ifdef BOOST_NO_UNREACHABLE_RETURN_DETECTION
+#ifndef BOOST_UNREACHABLE_RETURN
-#  define BOOST_UNREACHABLE_RETURN(x) return x;
+#  ifdef BOOST_NO_UNREACHABLE_RETURN_DETECTION
-#else
+#     define BOOST_UNREACHABLE_RETURN(x) return x;
-#  define BOOST_UNREACHABLE_RETURN(x)
+#  else
+#     define BOOST_UNREACHABLE_RETURN(x)
+#  endif
 #endif
 
 // BOOST_DEDUCED_TYPENAME workaround ------------------------------------------//