head files

what is the essencial head files needed to compile source code? gltools.h?

gl.h (for GL 1.1)
glext.h (for everything beyond 1.1)
wglext.h (for windows specific functions)
glxext.h (for *nix specific functions)

Or, just use an extension loader
http://www.opengl.org/wiki/OpenGL_Loading_Library