Drawing basic Geometrics

I’ve been doing a bit of research into drawing 2-D geometrics using OpenGL. I’ve found examples for circles by drawing repeated, connected triangles. However, all of the toolkits built on OpenGL I’ve seen that offer arc (or ellipse) drawing routines do something like this:

void drawArc(int x, int y, int w, int h) {
#ifdef _XWINDOWS

(where wDrawArc is whatever the Win32 command is).

I’m adapting the standard ellipse formula to draw with OpenGL draw commands, but was just wondering why there doesn’t seem to be a library to do these that is more cross-platform than using defines.