travis: test "make install" and "make dist" on macOS
    
    We can't use "make distcheck" on macOS because many unit tests fail. We
    can still get coverage of some of the things "distcheck" validates, by
    running the "install" and "dist" targets. This is particularly useful
    because many conditional features are disabled on macOS, and this helps
    make sure we can still successfully install & dist when these bits are
    disabled.
    
    The default script is getting unreadable since it is all on one long
    line. Rather than adding further conditional clauses to it, we make
    use of the travis matrix config override for the script.
    
    Reviewed-by: Andrea Bolognani <abologna@redhat.com>
    Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>