The -import-objc-header flag specifies a header for swiftc to import:

// 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)")

-import-objc-header defs.h