Ran into this error when compiling some software from source:
Could not find directory of OpenSSL installation, and this `-sys` crate cannot proceed without this knowledge. If OpenSSL is installed and this crate had trouble finding it, you can set the `OPENSSL_DIR` environment variable for the compilation process.
The fix is, luckily, rather easy.
Mac OSX / Homebrew
If you have Homebrew installed:
$ brew install firstname.lastname@example.org
Debian / Ubuntu
apt package manager:
$ apt install pkg-config libssl-dev
Fedora / RHEL / CentOS
$ pkg-config openssl-devel
Or with the newer
dnf package manager:
$ dnf install pkg-config openssl-devel