Implements #236

Most CMake configurations use the ON|OFF convention to indicate a boolean value. It would be more ergonomic to add a new setter like fn define_bool(&mut self, key: &str, value: bool). Some build.rs scripts could also compute these values during execution, so having this function would simplify

cfg.define("SOMETHING", if flag { "ON" } else { "OFF" });

into

cfg.define_bool("SOMETHING", flag);