* m4/gccwarn.m4: Use -Wdocumentation if supported.
* src/ltlast/binop.hh, src/ltlparse/public.hh, src/ta/taproduct.hh,
src/taalgos/emptinessta.hh, src/taalgos/reachiter.hh,
src/tgba/state.hh, src/tgba/tgbasafracomplement.cc,
src/tgbaalgos/ltl2tgba_fm.hh: Fix Doxygen documentations errors signaled
by clang++ 3.2.