Allow overriding lib/include dirs when cross-compiling.

fn main() {

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

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

    println!("target == {}", target);

    println!("host == {}", host);

    let mut env_lib_dir = "OPENSSL_LIB_DIR".to_string();

    let mut env_include_dir = "OPENSSL_INCLUDE_DIR".to_string();

    if target != host {

        env_lib_dir = "TARGET_OPENSSL_LIB_DIR".to_string();

        env_include_dir = "TARGET_OPENSSL_INCLUDE_DIR".to_string();

    }

    println!("cargo:rerun-if-env-changed=OPENSSL_LIB_DIR");

    let lib_dir = env::var_os("OPENSSL_LIB_DIR").map(PathBuf::from);

    println!("cargo:rerun-if-env-changed=OPENSSL_INCLUDE_DIR");

    let include_dir = env::var_os("OPENSSL_INCLUDE_DIR").map(PathBuf::from);

    println!("cargo:rerun-if-env-changed={}", env_lib_dir);

    let lib_dir = env::var_os(env_lib_dir).map(PathBuf::from);

    println!("cargo:rerun-if-env-changed={}", env_include_dir);

    let include_dir = env::var_os(env_include_dir).map(PathBuf::from);

    let (lib_dir, include_dir) = if lib_dir.is_none() || include_dir.is_none() {

        println!("cargo:rerun-if-env-changed=OPENSSL_DIR");