pub fn add_flags(&mut self, flags: impl IntoIterator<Item = impl Into<Cow<'static, str>>>) {
        self.flags.extend(flags.into_iter().map(|s| s.into()));
    }
Adds a set of flags to accept.

A flag is a single token like the `foo` in `cfg(not(foo))`.

A default `cargo build` will always evaluate flags to false, but custom wrappers may cause
some flags to evaluate to true. For example, as of version 0.6, `cargo web build` will cause
`cargo_web` to evaluate to true.
target-spec/src/platform.rs