introduce %if 0%{?use_system_re2} %if 0%{?use_system_jsoncpp} to select where to use system vs. shipped libs better
    
    make things a bit more optional for shipped vs. built-in libs re2 and jsoncpp with variables collected at one place.

    F30 fails to build because re2 is too small, centos-stream and rhel don't ship them at all while they are included in epel 8 already.