Suppress switch case fall-through warning

Since there are occasions where a switch case fall through is desirable,
GCC and Clang provide an attribute, __attribute__((fallthrough)), that
is to be used along with a null statement to suppress this warning that
would normally occur:

New macro "fallthrough" is defined according to the compilers used to
build this project.

Author: jserv

include/twin_private.h

@@ -12,7 +12,35 @@
 
 #include "twin.h"
 
+/* Boilerplate for compiler compatibility */
+#ifndef __has_attribute
+#define __has_attribute(x) 0
+#endif
+
+/* since C23 */
+#ifndef __has_c_attribute
+#define __has_c_attribute(x) 0
+#endif
+
+#if defined(__clang__) || defined(__GNUC__)
 #define maybe_unused __attribute__((unused))
+#else
+#define maybe_unused
+#endif
+
+/* suppress warnings when intentional switch case fall-through is desired. */
+#if defined(__has_c_attribute) && __has_c_attribute(fallthrough) /* C23+ */
+#define fallthrough [[fallthrough]]
+#elif defined(__has_attribute) && __has_attribute(fallthrough)
+/* GNU-style attribute. The __has_attribute() macro was first available in Clang
+ * 2.9 and incorporated into GCC since GCC 9.
+ */
+#define fallthrough __attribute__((fallthrough))
+#elif defined(__GNUC__) && __GNUC__ >= 7
+#define fallthrough __attribute__((__fallthrough__))
+#else
+#define fallthrough ((void) 0)
+#endif
 
 /*
  * Fixed-point type definitions

src/path.c

@@ -76,6 +76,7 @@ void _twin_path_smove(twin_path_t *path, twin_sfixed_t x, twin_sfixed_t y)
     switch (_twin_current_subpath_len(path)) {
     default:
         _twin_path_sfinish(path);
+        fallthrough;
     case 0:
         _twin_path_sdraw(path, x, y);
         break;

src/window.c

@@ -435,6 +435,7 @@ bool twin_window_dispatch(twin_window_t *window, twin_event_t *event)
     case TwinEventButtonUp:
         window->screen->button_x = -1;
         window->screen->button_y = -1;
+        return true;
     case TwinEventMotion:
         if (window->screen->button_x >= 0) {
             twin_coord_t x, y;