Updated potrace to 1.14.

February 19, 2017: Release 1.14

This release consists of bugfixes and minor portability improvements.
A number of bugs triggered by malformed BMP files have been fixed,
including CVE-2016-8685 and CVE-2016-8686. Error reporting has been
improved. The image size is now truncated when the bitmap data ends
prematurely. It is now possible to use negative dy in bitmap
data. Portability has been improved to encompass C++11. The default
compiler is now clang if available. Thanks to Nelson Beebe and Martin
Gieseking for reporting portability issues, and to Agostino Sarubbo
for reporting bugs.

(wiz)