converting ppk to pem on Linux

I had some PuTTY private key in .ppk format which I wanted to use on a Linux machine. One way would have been to install putty on Linux. Another way was to convert the ppk to pem format, and then use the standard OpenSSH ssh command. 

https://protocoderspoint.com/ubuntu-ssh-login-using-pem-file/

The version of puttygen available via the ubuntu repos seemed to be too old - so, downloaded and installed via https://www.chiark.greenend.org.uk/~sgtatham/putty/latest.html as per

puttygen: error loading x.ppk: PuTTY key format too new – ServerOK

So, 

tar -xvf putty*.tar.gz

cd putty[version]

# from the README

cmake .

cmake --build .

sudo cp puttygen /usr/bin/

and then 

puttygen server.ppk -O private-openssh -o server.pem