Check out stb_truetype.h, at the core it's a glyph rasterizer, but I guess the code to get at the outline must be in there somewhere and would be easy to extract (or by adding another API function).

https://github.com/nothings/stb/blob/master/stb_truetype.h

PS: looks like it's here:

https://github.com/nothings/stb/blob/f54acd4e13430c5122cab4c...