allow running pkg-config when targetting windows from non-windows hosts

/// typically tells us all the information that we need.

fn try_pkg_config() {

    let target = env::var("TARGET").unwrap();

    let host = env::var("HOST").unwrap();

    // If we're going to windows-gnu we can use pkg-config, but only so long as

    // we're coming from a windows host.

    //

    // Otherwise if we're going to windows we probably can't use pkg-config.

    if target.contains("windows-gnu") && host.contains("windows") {

    // If we're using mingw (windows-gnu*), we can use pkg-config, but we need

    // to allow mismatched host/target.

    if target.contains("windows-gnu") {

        env::set_var("PKG_CONFIG_ALLOW_CROSS", "1");

    } else if target.contains("windows") {

        // MSVC targets use vcpkg instead.

        return;

    }