The Weight property is ignored on Linux

[RazrFalcon]

extern crate font_kit;

fn main() {
    let names = vec![font_kit::family_name::FamilyName::Title("Arial".to_string())];
    let properties = font_kit::properties::Properties {
        style: font_kit::properties::Style::Normal,
        weight: font_kit::properties::Weight::BOLD,
        stretch: font_kit::properties::Stretch::NORMAL,
    };
    let handle = font_kit::source::SystemSource::new().select_best_match(&names, &properties).unwrap();
    println!("{:?}", handle); // Path { path: "/usr/share/fonts/corefonts/arial.ttf", font_index: 0 }
}

~> fc-match 'Arial :weight=700' --format="%{file}\n"
/usr/share/fonts/corefonts/arialbd.ttf

[RazrFalcon]

Works on Ubuntu 18.04, so it's probably a fontconfig's problem on my gentoo.

Will add a test case later.