Issue #500, reported by Yann Thierry-Mieg.

* spot/parsetl/parsetl.yy, spot/parsetl/scantl.ll: Use variant
to store a new pnode objects that delays the construction of n-ary
operators.
* spot/parsetl/Makefile.am: Do not distribute stack.hh anymore.
* spot/tl/formula.cc: Fix detection of overflow in Star and FStar.
* HACKING: Update Bison requirements to 3.3.
* tests/core/500.test: New test case.
* tests/Makefile.am: Add it.
* tests/core/ltl2tgba2.test, tests/core/ltlsynt.test,
tests/core/tostring.test: Adjust to new expected order.
* NEWS: Mention the change.