How to install libssl-dev on Slackware?

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http://unix.stackexchange.com – libssl-dev does not seem to be installed on my Slackware server and is needed for various dependencies. How can I install this on Slackware? Searching around did not find anything and there really isn't a package manager with Slackware. I did try slackpkg, but there are no libssl packages on there. I do already have openssl installed and have for a long time. Is this a separate package from OpenSSL? (HowTos)