graphics/potrace: Update to 1.15

Port changes:

• Changed DISTVERSION
• Simplified MASTER_SITES
• Added LIBRARY DOCS options
• Moved plist into pkg-plist
• Added CONFLICTS

PR: 221938
Submitted by: Piotr Smyrak <ps.ports@smyrak.com> (maintainer)
Approved by: tcberner (mentor, implicit)