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/defaultenv.cc

@@ -6,6 +6,10 @@ namespace spot
   namespace ltl
   {
 
+    default_environment::~default_environment()
+    {
+    }
+
     formula*
     default_environment::require(const std::string& s)
     {

src/ltlenv/defaultenv.hh

@@ -17,6 +17,7 @@ namespace spot
     class default_environment : public environment
     {
     public:
+      virtual ~default_environment();
       virtual formula* require(const std::string& prop_str);
       virtual const std::string& name();
 

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.
     };