Fix warnings from stringifying functions

The only way to stream those is to use the `bool` overload of `op<<`.
However, to convert a function to bool, GCC creates AST equivalent
of `A? true : false`. Then, because `A` is a function, it warns that
it will never be `false`. 🤦

As a bonus, newer GCC versions issue _two_ different warnings about
this, but older GCC versions do not know both of them, so we also
have to suppress warning about unknown warning suppression.

src/catch2/catch_stream.h

@@ -39,11 +39,27 @@ namespace Catch {
 
         auto str() const -> std::string;
 
+#if defined(__GNUC__) && !defined(__clang__)
+#pragma GCC diagnostic push
+// Old versions of GCC do not understand -Wnonnull-compare
+#pragma GCC diagnostic ignored "-Wpragmas"
+// Streaming a function pointer triggers Waddress and Wnonnull-compare
+// on GCC, because it implicitly converts it to bool and then decides
+// that the check it uses (a? true : false) is tautological and cannot
+// be null...
+#pragma GCC diagnostic ignored "-Waddress"
+#pragma GCC diagnostic ignored "-Wnonnull-compare"
+#endif
+
         template<typename T>
         auto operator << ( T const& value ) -> ReusableStringStream& {
             *m_oss << value;
             return *this;
         }
+
+#if defined(__GNUC__) && !defined(__clang__)
+#pragma GCC diagnostic pop
+#endif
         auto get() -> std::ostream& { return *m_oss; }
     };
 }