Take a look at this library -
The GTS Library[
^]
Quote -
GTS stands for the GNU Triangulated Surface Library. It is an Open Source Free Software Library intended to provide a set of useful functions to deal with 3D surfaces meshed with interconnected triangles. The source code is available free of charge under the Free Software LGPL license.
A brief summary of its main features:
...
2D dynamic Delaunay and constrained Delaunay triangulations.
...
LGPL allows commercial use.
But it has only limited support for Windows (whatever it means) -
From version 0.4.0, limited support has been added for the Microsoft windows environment and the Microsoft C compiler.