[Bro-Dev] #388: Fix more compiler warnings

[Jonathan Siwek]

> Could we turn a rebuild of the source code with the -Werror option into 
> a btest case, rather than actually using it for the initial build?

That's a neat idea, however, any use of -Werror is going to abort the build prematurely, right?  So we'll only be capturing the details of the first instance of a warning being emitted if we rely on -Werror, when I think it's more useful if NMI could give us all the warnings that were emitted.

But I'm still open to adding an additional btest that uses -Werror that would be useful at getting developers to think twice about filing a merge request for code that breaks tests because of warnings.

- Jon