* m4/gccwarn.m4: Add the option.
* bin/autfilt.cc, bin/common_output.hh, bin/dstar2tgba.cc,
bin/ltl2tgba.cc, bin/ltl2tgta.cc, bin/ltlcross.cc, bin/ltldo.cc,
bin/ltlfilt.cc, bin/ltlgrind.cc, spot/kripke/kripke.hh,
spot/ltsmin/ltsmin.cc, spot/ta/ta.hh, spot/ta/tgtaproduct.hh,
spot/taalgos/dot.cc, spot/taalgos/reachiter.hh,
spot/taalgos/statessetbuilder.cc, spot/taalgos/stats.cc,
spot/twa/twaproduct.cc, spot/twaalgos/emptiness.cc,
spot/twaalgos/gtec/ce.cc, spot/twaalgos/lbtt.cc,
spot/twaalgos/ndfs_result.hxx, spot/twaalgos/stats.hh,
spot/twaalgos/tau03opt.cc, tests/core/ngraph.cc: Add suggested override
qualifiers.