Fixes #106.

* m4/gccwarn.m4: Use -Wzero-as-null-pointer-constant if supported.
* src/bin/autfilt.cc, src/bin/common_aoutput.cc,
src/bin/common_finput.cc, src/bin/common_finput.hh,
src/bin/common_output.cc, src/bin/common_output.hh,
src/bin/common_post.cc, src/bin/common_r.hh, src/bin/common_range.hh,
src/bin/common_setup.cc, src/bin/common_trans.cc, src/bin/dstar2tgba.cc,
src/bin/genltl.cc, src/bin/ltl2tgba.cc, src/bin/ltl2tgta.cc,
src/bin/ltlcross.cc, src/bin/ltldo.cc, src/bin/ltlfilt.cc,
src/bin/ltlgrind.cc, src/bin/randaut.cc, src/bin/randltl.cc,
src/bin/spot-x.cc, src/kripke/kripkeexplicit.cc, src/ltlast/formula.cc,
src/ltlvisit/randomltl.cc, src/ltlvisit/randomltl.hh,
src/ltlvisit/relabel.cc, src/ltlvisit/relabel.hh,
src/ltlvisit/simplify.cc, src/ltlvisit/snf.hh, src/misc/fixpool.hh,
src/misc/mspool.hh, src/misc/hash.hh, src/misc/optionmap.cc,
src/misc/satsolver.cc, src/misc/tmpfile.cc, src/misc/tmpfile.hh,
src/priv/bddalloc.cc, src/ta/ta.hh, src/ta/taexplicit.cc,
src/ta/taexplicit.hh, src/ta/taproduct.cc, src/ta/tgtaexplicit.hh,
src/ta/tgtaproduct.cc, src/taalgos/minimize.cc,
src/taalgos/reachiter.cc, src/taalgos/tgba2ta.cc,
src/tests/complementation.cc, src/tests/ikwiad.cc,
src/tests/randtgba.cc, src/tests/reduc.cc, src/twa/bdddict.cc,
src/twa/bddprint.cc, src/twa/taatgba.cc, src/twa/twa.cc,
src/twa/twagraph.hh, src/twa/twaproduct.cc,
src/twa/twasafracomplement.cc, src/twaalgos/bfssteps.cc,
src/twaalgos/compsusp.cc, src/twaalgos/dtgbasat.cc,
src/twaalgos/emptiness.cc, src/twaalgos/gtec/ce.cc,
src/twaalgos/gv04.cc, src/twaalgos/ltl2taa.cc,
src/twaalgos/ltl2tgba_fm.cc, src/twaalgos/ltl2tgba_fm.hh,
src/twaalgos/magic.cc, src/twaalgos/minimize.cc,
src/twaalgos/ndfs_result.hxx, src/twaalgos/postproc.cc,
src/twaalgos/postproc.hh, src/twaalgos/reachiter.cc,
src/twaalgos/reducerun.cc, src/twaalgos/safety.hh,
src/twaalgos/sccfilter.hh, src/twaalgos/se05.cc,
src/twaalgos/simulation.cc, src/twaalgos/tau03.cc,
src/twaalgos/tau03opt.cc, src/twaalgos/translate.cc,
src/twaalgos/translate.hh, utf8/utf8/core.h,
bench/stutter/stutter_invariance_formulas.cc, iface/ltsmin/ltsmin.cc,
iface/ltsmin/modelcheck.cc: Use nullptr to fix the warning.