Fix compiler warnings and enable -Werror

Currently compiler warnings are being ignored. This masks a few bad coding practices, and makes it difficult to verify that new code is working correctly. Once we've fixed all of them, we can add -Werror to prevent us from introducing new warnings.

Most are easy fixes, such as removing unused variables and adding function signatures to *.h files. Others are caued by the use of deprecated, or misuse of common functions. Typically the warning itself mentions the best fix.

Fixing this will make tasks like the gstreamer upgrade much easier to verify.