I found this error message when compiling some software from source:
It looks like you're compiling on Linux and also targeting Linux. Currently this requires the `pkg-config` utility to find OpenSSL but unfortunately `pkg-config` could not be found. If you have OpenSSL installed you can likely fix this by installing `pkg-config`.
Luckily, it’s an easy fix!
$ apt install pkg-config
This’ll install the required binary:
$ which pkg-config /usr/bin/pkg-config
And you’re all set!