MacのターミナルでObjective-Cをコンパイルする方法 ref: http://qiita.com/ikuwow/items/5470ffea3bf7403fe0a7

file2.txt

$ gcc -c main.m

file3.txt

$ gcc -c main.m
$ gcc -c Song.m
$ gcc -c Singer.m
$ gcc -o myFirstProgram main.o Song.o Singer.o -framework Foundation

main.m

#import "Song.h"
#import "Singer.h"

int main(void) {

    id song;
    id singer;

    song = [[Song alloc] init];
    [song setLyrics:@"それが僕達の奇跡"];

    singer = [[Singer alloc] init];
    [singer setSong:song];
    [singer sing];

    return 0;
}

Singer.m

#import <Foundation/NSObject.h>
#import <stdio.h>
（略）