Increase CMake required version 3.5 -> 3.20

[GabrielGanne]

Hi,

Commit 777e0b245c510e273e5ba917939a21778cb060f2 introduced cmake_path which has been added to Cmake in 3.20 only (https://cmake.org/cmake/help/git-master/command/cmake_path.html)

This should only make the error message more explicit.

Best regards,