app, app-tools: Fix WinMain definition on some executables

Now, it works on both GNU compilers and MSVC.

app-tools/gimp-debug-tool.c

@@ -121,3 +121,14 @@ main (int    argc,
 
   exit (EXIT_SUCCESS);
 }
+
+#ifdef G_OS_WIN32
+int WINAPI
+WinMain (HINSTANCE hInstance,
+         HINSTANCE hPrevInstance,
+         LPSTR     lpCmdLine,
+         int       nCmdShow)
+{
+  return main (__argc, __argv);
+}
+#endif

app/main.c

@@ -918,17 +918,11 @@ main (int    argc,
  * used on MSVC builds only.
  */
 
-#ifdef __GNUC__
-#  ifndef _stdcall
-#    define _stdcall  __attribute__((stdcall))
-#  endif
-#endif
-
-int _stdcall
-WinMain (struct HINSTANCE__ *hInstance,
-         struct HINSTANCE__ *hPrevInstance,
-         char               *lpszCmdLine,
-         int                 nCmdShow)
+int WINAPI
+WinMain (HINSTANCE hInstance,
+         HINSTANCE hPrevInstance,
+         LPSTR     lpCmdLine,
+         int       nCmdShow)
 {
   return main (__argc, __argv);
 }