Pick the appropriate legacy/setup-*.sh for your system and run it.
On Linux systems you may need to run the script with sudo.
Please read the scripts carefully and make sure you agree with the changes to your system!!
These scripts are maintained on a best-effort basis. If you find something missing or not working, please describe your case by opening an issue and let us discuss a solution together.
If your system does not provide the required CMake version, you may run
<path-to-source>/bootstrap-cmake.sh <install-dir>
export PATH=<install-dir>/bin:$PATH
to install a recent CMake version to an <install-dir> of your choice.
<path-to-source> is the path to your FairSoft git clone.