November 24, 2016; Ulf Griesmann
- update clipper library to version 6.4
- add functions 'polyarea' and 'polycentr'.

September 2, 2014; Ulf Griesmann
- version 23
- fix a small error in the help text
- cast arguments mx* functions to mwSize and mwIndex as
  needed to make function 64-bit proof.

August 30, 2014; Ulf Griesmann
- version 22
- this version is based on the Clipper library
  by Angus Johnson.

June 5, 2014; Ulf Griesmann
- version 21
- update to recently released version 2.33 of the GPC library
- various cleanups
