Compiling a C program that uses OpenGl in Mac OS X

I am trying to compile a C program for my CS class. I have Command Line tools installed on my Mac, so I probably have OpenGL. The program description was made for Ubuntu and it says for me to compile using:

gcc -Wall -ansi -pedantic -O2 main.o graphic.o imagem.o io.o -o ep2 -lGL -lGLU -lglut

I ran that and it said:

ld: library not found for -lGL

What flags should I use? What do I do?