Move to CFLAG=-pedantic-errors

[evanmcc]

since it's the default on the erlang side, we should be doing whatever static analysis we can on the C side. We can also consider going to C99 and look at places where that might make the code cleaner.

I am assigning this to 2.1 because there's some erlang/rebar magic going on that makes "erl_nifs.h" not findable at check time. If there's a simple solution to that (I haven't even googled) then we might consider making this 2.0-final or 2.0.1, although if we do that, we should make going to C99 another issue, since that's too much change right now.

To do this, all that we have to do is add {"DRV_CFLAGS", "-pedantic-errors"}, or {"DRV_CFLAGS", "-pedantic-errors" -std=c99}, to rebar.config, and do the required cleanup.