Cannot start xDAQ daq04 after merge request !162

Description

If one compiles without explicitly setting the installation prefix, xDAQ crashes at start with an error like the following:

Caught exception: xdaq::exception::ConfigurationError 'Cannot load module from [omissis]/cmsgemos/_build/_install/lib64/libgemsupervisor.so' raised at downloadModules(/gitlab-ci_git_clone_and_software_build_directory/cmsos/core/executive/src/common/Application.cc:1320);

I believe this is due to GNUInstallDirs being included in the CMakeLists.txt file before the default value for CMAKE_INSTALL_PREFIX is set (change introduced in MR !162 (merged)).