Fix CFLAGS options in ext/json/ext/{generator,parser}/extconf.rb

1. [generator/extconf.rb](https://github.com/flori/json/blob/18261880e33b9bd70152d860ea88c784ebef07e4/ext/json/ext/generator/extconf.rb#L8-10) enables `-O0  -ggdb` when $DEBUG is _false_.
2. When $CFLAGS contains <code> -Ofast</code>, extconf.rb replaces it with `-O3fast`, which is invalid and causes compile error.

This patch fixes these two problems.