Fix some C compile warnings for Ruby

This fixes the warnings coming out of ast.c using
the fixes here:
http://stackoverflow.com/questions/3599160/unused-parameter-warnings-in-c-code

However I can't see how to fix the warnings coming from
lexer.c since it seems that code is auto-generated by
flex.
Tweak the lexer with sed to remove warnings. Still getting some 'ISO C90 forbids mixed declarations and code' warnings that we need to fix.
Fixed warnings on both clang and gcc. This shoulw really fix #3
Great. sed -i behaves differently on OS X and Linux
Merge warning fixes