add warnings as errors to the nif build flags

Turns out that the rebar problems I was seeing is because rebar uses the somewhat brittle-feeling strategy of replacing all of the default DRV_CFLAGS with the ones that you specify, so you have to add all of them to get any changes made.  I reformatted everything because one of the lines was long.

I played with various pedantic and -std modes, but the scope of changes didn't seem worth it, especially for pedantic.

Fixes #159