Introduce build-condition for JSON::MaybeXS test
    
    The tests fall back to JSON::PP if JSON::MaybeXS is not available.
    
    Reason for thus condition is this building autotools: JSON::MaybeXS
    requires Compress::LZF that requires liblzf that requires autotools.
    And autotools require modules whose packages are built by
    Module::Install that requires YAML::Tiny that goes back JSON::MaybeXS
    tests.