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

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

source/blender/python/mathutils/mathutils_Matrix.c

@@ -2034,7 +2034,7 @@ static PyObject *Matrix_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;