Compile with -Wmissing-declarations

[paulharris]

Hi,

Your docs say you compile with a lot of warnings enabled, with warnings treated as errors, that's great,

have you considered also adding -Wmissing-declarations? flatbuffers won't compile with that flag added.

cheers :)

[paulharris]

Have you also considered: -Wzero-as-null-pointer-constant

flatc generates code with zeros instead of NULL or nullptr, which is throwing up warnings when I compile in the generated headers.

Would be nice if it wasn't using zero, especially for C++17 mode...

[CasperN]

I'd be happy with maintaining that, would you like to contribute a PR for it?