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.