autostart: Enable setting via cmake Exec= path

See https://github.com/NixOS/nixpkgs/issues/199596 .

widgets/CMakeLists.txt

@@ -116,6 +116,12 @@ if (SYNCTHING_WIDGETS_LOG_JAVASCRIPT_CONSOLE)
     message(WARNING "JavaScript console of web view will be logged to stderr")
 endif ()
 
+# configure autostart .desktop file exec path
+option(AUTOSTART_EXEC_PATH "Set the Exec= path for Linux' autostart/syncthingtray.desktop file - useful for Nix & Guix")
+if (AUTOSTART_EXEC_PATH)
+    add_compile_definitions(AUTOSTART_EXEC_PATH="${AUTOSTART_EXEC_PATH}")
+endif()
+
 # link also explicitly against the following Qt modules
 list(APPEND ADDITIONAL_QT_MODULES Network Concurrent)
 

widgets/settings/settingsdialog.cpp

@@ -802,7 +802,11 @@ bool setAutostartEnabled(bool enabled)
         desktopFile.write("[Desktop Entry]\n"
                           "Name=" APP_NAME "\n"
                           "Exec=\"");
+#if defined(AUTOSTART_EXEC_PATH)
+        desktopFile.write(qEnvironmentVariable("APPIMAGE", AUTOSTART_EXEC_PATH).toUtf8().data());
+#else
         desktopFile.write(qEnvironmentVariable("APPIMAGE", QCoreApplication::applicationFilePath()).toUtf8().data());
+#endif
         desktopFile.write("\" qt-widgets-gui --single-instance\nComment=" APP_DESCRIPTION "\n"
                           "Icon=" PROJECT_NAME "\n"
                           "Type=Application\n"