Fix errors reported by ICC.

* src/tgba/state.hh (state_ptr_less_than::operator()): Make it const.
* src/tgba/tgbaproduct.cc: Include string.hh.
* src/ltlast/multop.hh (multop::add, multop::add_sorted): Do
not use qualified names in declarations.
* m4/gccwarn.m4 (CF_GXX_WARNINGS): Fix GXX test.
* src/ltlenv/defaultenv.hh, src/ltlenv/defaultenv.cc,
src/ltlenv/environment.hh: Add virtual destructors.

src/ltlenv/environment.hh

@@ -35,6 +35,11 @@ namespace spot
       /// Get the name of the environment.
       virtual const std::string& name() = 0;
 
+      virtual
+      ~environment()
+      {
+      };
+
       // FIXME: More functions will be needed later, but
       // it's enough for now.
     };