I think that 'CONFLICTS_BUILD=spidermonkey17-1.7' should be removed. I don't think it is correct. If I remove that CONFLICTS_BUILD line, install spidermonkey17 and build (and install) cjs, the build does not fail. And I don't detect any problem with either installed port. Note that the CONFLICTS_BUILD line has been there from the beginning, so the commit log offers no hints about why it was added.
Created attachment 206347 [details] svn-diff-cjs
Reset Maintainer. Two years without reaction.
A commit references this bug: Author: danfe Date: Tue Oct 27 08:11:24 UTC 2020 New revision: 553424 URL: https://svnweb.freebsd.org/changeset/ports/553424 Log: GC outdated conflicts: lang/njs was removed on 2011-06-09 and CONFLICTS_BUILD=spidermonkey17 does not reflect reality these days, if it ever did. PR: 222289 Changes: head/lang/cjs/Makefile head/lang/spidermonkey17/Makefile head/lang/spidermonkey185/Makefile head/lang/spidermonkey24/Makefile
I also couldn't see how it's useful and GC'ed it as part of ports r553424.