Explicitly annotate intentional switch fallthrough

This silences a very useful warning in GCC 10.

config_cmake.h.in

@@ -198,3 +198,9 @@
 #else
 #define __warn_unused_type
 #endif
+
+#if __has_attribute(fallthrough)
+#define __fallthrough__ __attribute__ ((fallthrough))
+#else
+#define __fallthrough__
+#endif

src/reader.cpp

@@ -2941,9 +2941,9 @@ void reader_data_t::handle_readline_command(readline_cmd_t c, readline_loop_stat
                 parser().libdata().is_repaint = false;
                 break;
             }
-            // Else we repaint as normal
+            // Else we repaint as normal.
+            __fallthrough__
         }
-            /* fallthrough */
         case rl::force_repaint:
         case rl::repaint: {
             parser().libdata().is_repaint = true;