Improve detection of std::uncaught_exceptions support

The problem was that Catch2 did not reliably include `<exception>`
before it checked for the feature test macro for
`std::uncaught_exceptions`. To avoid overhead of including
`<exception>` everywhere, the configuration check was split out
into a separate header.

Closes #2021

include/internal/catch_uncaught_exceptions.cpp

@@ -8,6 +8,8 @@
 
 #include "catch_compiler_capabilities.h"
 #include "catch_uncaught_exceptions.h"
+#include "catch_config_uncaught_exceptions.hpp"
+
 #include <exception>
 
 namespace Catch {