BoundingBox support for Lines,

BoundingBox constructor will no more throw for empty vector of points.

GMP allowed for Vojtech's fork of boost::polygon Voronoi implementation.

Added libslic3r tests for boost::polygon Voronoi. All Voronoi issues
ever reported on the Internet are captured by the tests. Two issues
reported (the two test cases) are real issues which may influence
PrusaSlicer negatively, namely

https://github.com/boostorg/polygon/issues/43

src/libslic3r/Line.hpp

@@ -103,6 +103,8 @@ public:
     Vec3d b;
 };
 
+extern BoundingBox get_extents(const Lines &lines);
+
 } // namespace Slic3r
 
 // start Boost