Silence g++7 switch case fallthrough warnings

src/sass_types/color.cpp

@@ -28,7 +28,7 @@ namespace SassTypes
       }
 
       a = Nan::To<double>(raw_val[3]).FromJust();
-      // fall through vvv
+      NODE_SASS_FALLTHROUGH;
 
     case 3:
       if (!raw_val[0]->IsNumber() || !raw_val[1]->IsNumber() || !raw_val[2]->IsNumber()) {

src/sass_types/color.h

@@ -4,6 +4,12 @@
 #include <nan.h>
 #include "sass_value_wrapper.h"
 
+#if defined(__GNUC__) && __GNUC__ >= 7
+#define NODE_SASS_FALLTHROUGH __attribute__ ((fallthrough))
+#else
+#define NODE_SASS_FALLTHROUGH
+#endif
+
 namespace SassTypes
 {
   class Color : public SassValueWrapper<Color> {