UG and IPOpt flags are both registered via
dune_register_package_flags(). Hence they
need not be supplied if any of

    dune_target_enable_all_packages()
    dune_enable_all_packages()

is used. The dune_add_test() function, however,
calls add_dune_all_flags() which is the predecessor
of dune_target_enable_all_packages() and has the
same effect. Hence we can drop all conditionals,
lists, and loop here.