Pierre Ganty wrote that he could not compile Spot with Swig 4.2.1
anymore, and when I upgraded from 4.2.0 to 4.2.1 I could not either.

It seems that declaring operator<< as friends in subclasses is
confusing Swig 4.2.1.  See https://github.com/swig/swig/issues/2845

* spot/twa/acc.cc, spot/twa/acc.hh: Declare operator<< for
acc_cond::mark_t and acc_cond::acc_code outside the class, so that we
do not need friend declarations.