Fix build warnings

I was getting a Makefile-based build going, and I always like to turn on -Werror when possible to help keep things tidy. This just cleans up a few build warnings (using 'clang version 3.2 (tags/RELEASE_32/final)').

There appear to be several other similar warnings issued from the core cinder libs - if you have any interest in cleaning those as well, I can volunteer a bit of time to help scrub them :)