security/botan2: update to 2.8.0

• Bump PORTREVISION for dependent ports
• While here do some cleanup to the Makefile.

ChangeLog: https://botan.randombit.net/news.html#version-2-8-0-2018-10-01

PR: 231899
Submitted by: tremere@cainites.net
Approved by: tcberner (mentor)
Differential Revision: https://reviews.freebsd.org/D17413