Hide std::exception_ptr and friends if exceptions disabled

Some compilers, e.g. the Green Hills C++ compiler, react badly to the
appearance of std::exception_ptr, std::current_exception,
std::rethrow_exception and std::uncaught_exception(s). To allow usage of
Catch2 with these compilers when exceptions are disabled, hide the usage
of std::exception_ptr etc. when compiling with
CATCH_CONFIG_DISABLE_EXCEPTIONS.

include/internal/catch_startup_exception_registry.h

@@ -15,11 +15,13 @@
 namespace Catch {
 
     class StartupExceptionRegistry {
+#if !defined(CATCH_CONFIG_DISABLE_EXCEPTIONS)
     public:
         void add(std::exception_ptr const& exception) noexcept;
         std::vector<std::exception_ptr> const& getExceptions() const noexcept;
     private:
         std::vector<std::exception_ptr> m_exceptions;
+#endif
     };
 
 } // end namespace Catch