graphics/aseprite: Fix conflict with textproc/cmark

Link with libcmark from textproc/cmark instead of using the embedded
cmark version and stop installing files that textproc/cmark already
provides.

PR: 222787
Reported by: rodrigo
Approved by: Yuri Victorovich <yuri@rawbw.com> (maintainer)
MFH: 2017Q4