current_accepting_conditions): Mark as const.
* src/tgba/succiterconcrete.cc, src/tgba/succiterconcrete.hh,
src/tgba/tgbaexplicit.cc, src/tgba/tgbaexplicit.hh,
src/tgba/tgbaproduct.cc, src/tgba/tgbaproduct.hh,
src/tgba/tgbatranslateproxy.cc, src/tgba/tgbatranslateproxy.hh:
Likewise.