Remediate compiler warnings during polytracker build, and enable `-Werror` build mode

[bradlarsen]

We currently encounter a few build-time warnings when building polytracker. We should remediate them -- either change the code in question, or determine that the particular warning type isn't useful to us and disable it.

We should also consider adding -Werror to the default polytracker build configuration, so that we don't introduce new warnings. To make this usable, however, we would need to additionally add a CMake build option to not add -Werror to the build configuration. (Without a mechanism like this, -Werror is hostile toward non-developer users who simply want to build polytracker from source, and may be using a different compiler from developers.)