Support common boolean convention of defining ON|OFF values

[nyurik]

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);

Workaround

Without this, I had to add this to all projects that needed this functionality:

trait CfgBool {
    fn define_bool(&mut self, key: &str, value: bool);
}

impl CfgBool for cmake::Config {
    fn define_bool(&mut self, key: &str, value: bool) {
        self.define(key, if value { "ON" } else { "OFF" });
    }
}