Log Message:
Update potrace to 1.15.

v1.15 2017/08/05
	(2017/08/05) PS1 - use round() instead of rint()
	(2017/08/05) PS1 - portability: set both _POSIX_C_SOURCE and
	_NETBSD_SOURCE to enable strdup prototype in string.h, as
	_XOPEN_SOURCE=500 doesn't work on macOS, _XOPEN_SOURCE=600 doesn't
	work on SunOS, and _POSIX_C_SOURCE alone doesn't work on Minix.
	(2017/08/02) PS1 - fixed a potential arithmetic overflow.
	(2017/06/17) PS1 - fixed a bug with very large bitmaps. Reported
	by Michael Voříšek.
	(2017/03/05) PS1 - autoconf: added --enable-local-getopt to avoid
	using the system-wide getopt library (it may still end up being
	used on GNU systems).
	(2017/03/05) PS1 - portability: moved getopt.h to a subdirectory,
	to be included only when needed. Reported by Daniel Macks.
	(2017/03/05) PS1 - portability: use _XOPEN_SOURCE=600 to fix macOS
	warning.
	(2017/02/27) PS1 - fixed more buffer overflows (not security
	critical), including one reported by Agostino Sarubbo. This also
	fixes CVE-2017-12067.