Installing Catch2 when using FetchContent

Due to the behaviors documented in https://github.com/catchorg/Catch2/issues/1373 Catch doesn't install anything when used as a subproject but this doesn't allow it to be installed when used through the FetchContent macro, that requires the package to be added with the add_subdirectory.

Could be useful to add an option, maybe even mark as advanced if necessary, to force the installation if the user wants to, even when used as subproject