Refactor all math into a new kimath library

* Split up the thirdparty code into the thirdparty folder (#3637)
* Create a new kimath static library containing all the math functions

This is part of cleaning the build system for #1906.

common/plotters/HPGL_plotter.cpp

@@ -203,6 +203,8 @@
 #include <macros.h>
 #include <kicad_string.h>
 #include <convert_basic_shapes_to_polygon.h>
+#include <math/util.h>      // for KiROUND
+
 
 // The hpgl command to close a polygon def, fill it and plot outline:
 // PM 2; ends the polygon definition and closes it if not closed