Add support for -fno-exceptions (or equivalent)

This means

* Adding new configuration toggle `CATCH_CONFIG_DISABLE_EXCEPTIONS`
and a best-guess configuration auto-checking for it.
* Adding new set of internal macros, `CATCH_TRY`, `CATCH_CATCH_ALL`
and `CATCH_CATCH_ANON` that can be used in place of regular `try`,
`catch(...)` and `catch(T const&)` respectively, while disappearing
when `CATCH_CONFIG_DISABLE_EXCEPTIONS` is enabled.
* Replacing all uses of `throw` with calls to `Catch::throw_exception`
customization point.
* Providing a default implementation for the above customization point
when `CATCH_CONFIG_DISABLE_EXCEPTIONS` is set.
* Letting users override this implementation with their own.
* Some minor changes and ifdefs all around to support the above

include/internal/catch_enforce.cpp

@@ -0,0 +1,19 @@
+/*
+ *  Created by Martin on 03/09/2018.
+ *
+ *  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_enforce.h"
+
+namespace Catch {
+#if defined(CATCH_CONFIG_DISABLE_EXCEPTIONS) && !defined(CATCH_CONFIG_DISABLE_EXCEPTIONS_CUSTOM_HANDLER)
+    [[noreturn]]
+    void throw_exception(std::exception const& e) {
+        Catch::cerr() << "Catch will terminate because it needed to throw an exception.\n"
+                      << "The message was: " << e.what() << '\n';
+        std::terminate();
+    }
+#endif
+} // namespace Catch;