CMake: Use GCC7's -Wimplicit-fallthrough=5

Use to avoid accidental missing break statements,
use ATTR_FALLTHROUGH to suppress.

source/blender/python/mathutils/mathutils_Euler.c

@@ -65,7 +65,7 @@ static PyObject *Euler_new(PyTypeObject *type, PyObject *args, PyObject *kwds)
 		case 2:
 			if ((order = euler_order_from_string(order_str, "mathutils.Euler()")) == -1)
 				return NULL;
-			/* fall-through */
+			ATTR_FALLTHROUGH;
 		case 1:
 			if (mathutils_array_parse(eul, EULER_SIZE, EULER_SIZE, seq, "mathutils.Euler()") == -1)
 				return NULL;
@@ -370,7 +370,7 @@ static PyObject *Euler_richcmpr(PyObject *a, PyObject *b, int op)
 	switch (op) {
 		case Py_NE:
 			ok = !ok;
-			/* fall-through */
+			ATTR_FALLTHROUGH;
 		case Py_EQ:
 			res = ok ? Py_False : Py_True;
 			break;