devel/gecode: Update from 5.0.0 to 5.1.0

[joneum]

Summary

• devel/gecode: Update from 5.0.0 to 5.1.0
• Changelog: http://www.gecode.org/doc-latest/reference/PageChange.html#SectionChanges_5_1_0

PR: 221083
Submitted by: Demetris Procopiou <[e-mail]> (maintainer)
Approved by: xxx (mentor)
#Differential Revision: https://reviews.freebsd.org/Dxxxxx

Test Plan

:~/dev/ports/devel/gecode % portlint -AC
WARN: Makefile: for new port, make $FreeBSD$ tag in comment section empty, to make SVN happy.
0 fatal errors and 1 warning found.

poudriere testport:

10.3-amd64
10.3-i386
11.1-amd64
11.1-i386
12.0-CURRENT r320266 amd64
12.0-CURRENT r320266 i386

[tz]

BTW: you did a build test for 11.1 but not for 11.0. Second one is still supported for two months