They are located at /System/Library/Frameworks/OpenGL.framework/Headers. To include them, just use:
#include <OpenGL/gl.h>
#include <OpenGL/glu.h>
#include <OpenGL/glext.h>
#include <GLUT/glut.h>
etc. Make sure to link against the appropriate frameworks, eg)
cc <your_file.c> -framework GLUT -framework OpenGL
for OpenGL and GLUT
与恶龙缠斗过久,自身亦成为恶龙;凝视深渊过久,深渊将回以凝视…