getdns 1.7.2 no longer builds on Ubuntu 20.04 LTS

[RMerl]

getdns fails to build on my Ubuntu 20.04 LTS dev system following commit 777e0b2 :

Unknown CMake command "cmake_path"

This is because cmake_path used in this commit was only added in cmake 3.20, and Ubuntu 20.04 LTS ships with 3.16.

[wtoorop]

Ack, I reopened Issue #517 which was the issue causing this dependency on this newer version of cmake, but which also has an alternative way to deal with it documented.