Added a workaround for uncaught_exceptions for older Mac OS and iOS.

libc++ disables std::uncaught_exceptions for Mac OS < 10.12 and
iOS < 10.0, even though it defines __cpp_lib_uncaught_exceptions.

Fixes boostorg#80.

include/boost/core/uncaught_exceptions.hpp

@@ -1,5 +1,5 @@
 /*
- *             Copyright Andrey Semashev 2018.
+ *          Copyright Andrey Semashev 2018 - 2020.
  * Distributed under the Boost Software License, Version 1.0.
  *    (See accompanying file LICENSE_1_0.txt or copy at
  *          https://www.boost.org/LICENSE_1_0.txt)
@@ -26,8 +26,23 @@
 #pragma once
 #endif
 
-// Visual Studio 14 supports N4152 std::uncaught_exceptions()
-#if (defined(__cpp_lib_uncaught_exceptions) && __cpp_lib_uncaught_exceptions >= 201411) || \
+#if defined(__APPLE__)
+#include <Availability.h>
+// Apple systems only support std::uncaught_exceptions starting with specific versions:
+// - Mac OS >= 10.12
+// - iOS >= 10.0
+// - tvOS >= 10.0
+// - watchOS >= 3.0
+// https://github.com/boostorg/core/issues/80
+#if (defined(__cpp_lib_uncaught_exceptions) && __cpp_lib_uncaught_exceptions >= 201411) && \
+    ( \
+        (defined(__MAC_OS_X_VERSION_MIN_REQUIRED) && __MAC_OS_X_VERSION_MIN_REQUIRED >= 101200) || \
+        (defined(__IPHONE_OS_VERSION_MIN_REQUIRED) && __IPHONE_OS_VERSION_MIN_REQUIRED >= 100000) \
+    )
+#define BOOST_CORE_HAS_UNCAUGHT_EXCEPTIONS
+#endif
+// Visual Studio 14.0 supports N4152 std::uncaught_exceptions() but doesn't define __cpp_lib_uncaught_exceptions
+#elif (defined(__cpp_lib_uncaught_exceptions) && __cpp_lib_uncaught_exceptions >= 201411) || \
     (defined(_MSC_VER) && _MSC_VER >= 1900)
 #define BOOST_CORE_HAS_UNCAUGHT_EXCEPTIONS
 #endif