Redirect std::uncaught_exception to Catch::uncaught_exception

This means that only one place needs to work with warnings from
the deprecation of `std::uncaught_exception()` in C++17.

Closes #1162.

include/internal/catch_uncaught_exceptions.cpp

@@ -0,0 +1,22 @@
+/*
+ *  Created by Josh on 1/2/2018.
+ *  Copyright 2018 Two Blue Cubes Ltd. All rights reserved.
+ *
+ *  Distributed under the Boost Software License, Version 1.0. (See accompanying
+ *  file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
+ */
+
+#include "catch_compiler_capabilities.h"
+#include "catch_uncaught_exceptions.h"
+#include <exception>
+
+namespace Catch {
+    bool uncaught_exceptions() {
+// https://github.com/catchorg/Catch2/issues/1162
+#if defined(CATCH_CONFIG_CPP17_UNCAUGHT_EXCEPTIONS)
+        return std::uncaught_exceptions() > 0;
+#else
+        return std::uncaught_exception();
+#endif
+  }
+} // end namespace Catch