audio/dssi: Add LICENSE_FILE, Use OPTIONS helpers, Add missing x11 dependency

[ultima]

Summary

• Add license file
• Cleanup Makefile
• Bump PORTREVISION for added USE_XORG depend

PR\: 221239
Submitted by\: Yuri Victorovich
Reviewed by\: lifanov (mentor), matthew (mentor)
Approved by\: lifanov (mentor), matthew (mentor)
MFH: 2017Q3
Differential Revision\: https://reviews.freebsd.org/DXXXXX

Is adding USE_XORG just a metadata change and therefore should not have revision bump?

Test Plan

portlint:
WARN: Makefile: for new port, make $FreeBSD$ tag in comment section empty, to make SVN happy.
WARN: Makefile: new ports should not set PORTREVISION.
WARN: Makefile: new ports should not be maintained by [e-mail].
0 fatal errors and 3 warnings found.

poudriere:
103i386
103amd64
110i386
110amd64
12i386
12amd64

[matthew]