build: re-enable compiler warnings, and fix them

There are warnings (-Wextra) in generated C++ code:

  ltlparse.cc: In member function 'ltlyy::parser::symbol_number_type
  ltlyy::parser::by_state::type_get() const':
  ltlparse.cc:452:33: warning: enumeral and non-enumeral type in
  conditional expression
      return state == empty_state ? empty_symbol : yystos_[state];

Reported by Alexandre Duret-Lutz.

It turns out that -Wall and -Wextra were disabled because of a stupid
typo.

* configure.ac: Fix the stupid typo.
* data/lalr1.cc, src/AnnotationList.c, src/InadequacyList.c,
* src/ielr.c, src/print.c, src/scan-code.l, src/symlist.c,
* src/symlist.h, src/symtab.c, src/tables.c, tests/actions.at,
* tests/calc.at, tests/cxx-type.at, tests/glr-regression.at,
* tests/named-refs.at, tests/torture.at:
Fix warnings, mostly issues about variables used only with assertions,
which are disabled with -DNDEBUG.