Also try PREFER_PKGSRC += openssl, but read the guide to find the right variable name and syntax. ALso, if you're trying to get the security fix, you could upgrade your netbsd 9 system along the branch. While releases are extra tested, I've been randomly updating along stable branches for years (15?) and do not remember a problem with a build that completed. (There have been a few failures to build, but those are every few years.)
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