为 swiftc 指定桥接头

-import-objc-header 标志指定要导入 swiftc 的标头：

// defs.h
struct Color {
    int red, green, blue;
};

#define MAX_VALUE 255

// demo.swift
extension Color: CustomStringConvertible {  // extension on a C struct
    public var description: String {
        return "Color(red: `(red)`, green: `(green)`, blue: `(blue)`)"
    }
}
print("MAX_VALUE is: `(MAX_VALUE)`")  // C macro becomes a constant
let color = Color(red: 0xCA, green: 0xCA, blue: 0xD0)  // C struct initializer
print("The color is `(color)`")

$ swiftc demo.swift -import-objc-header defs.h && ./demo
MAX_VALUE is: 255
The color is Color(red: 202, green: 202, blue: 208)