Hi,
I just modified a test program taken from AGG's examples to test the polygon clipping capability of following libraries.
*Boost.Polygon
*GPC
*Clipper

Here are some observations:
1) The proccesing speed (fastest to slowest): Clipper > GPC > Boost.Polygon
2) With "xor", although the results seems the same, they have different countours numbers (Clipper the least).
3) Boost.Polygon has a strange behavior that it sometimes produces erroneous vertical lines within the polygons.
In this example, with "or" operation you'll see that.

I put the test files here:
http://tinyurl.com/2axoqxc

Anyone who wants to compile it will also need AGG and Clipper on your computer.