0029151: GCC 7.1 warnings "this statement may fall through" [-Wimplicit-fallthrough=]

New macro Standard_FALLTHROUGH is defined for use in a switch statement immediately before a case label, if code associated with the previous case label may fall through to that
next label (i.e. does not end with "break" or "return" etc.).
This macro indicates that the fall through is intentional and should not be diagnosed by a compiler that warns on fallthrough.

The macro is inserted in places that currently generate such warning message and where fallthrough is intentional.

Doxygen comments are provided for this and other macros in Standard_Macro.hxx.

src/Intrv/Intrv_Intervals.cxx

@@ -194,9 +194,11 @@ void Intrv_Intervals::Unite (const Intrv_Interval& Tool)
     case Intrv_Similar :
       Tins.FuseAtStart(myInter(index).Start(),
 		       myInter(index).TolStart());    // modifier le debut
+      Standard_FALLTHROUGH
     case Intrv_JustEnclosingAtEnd :
       Tins.FuseAtEnd  (myInter(index).End  (),
 		       myInter(index).TolEnd  ());    // modifier la fin
+      Standard_FALLTHROUGH
     case Intrv_Enclosing :
       myInter.Remove(index);                          // detruire et
       index--;                                        // continuer