autostart: Enable setting via cmake Exec= path

[doronbehar]

With this patch, I managed to make syncthingtray create the file:

[Desktop Entry]
Name=Syncthing Tray
Exec="/run/current-system/sw/bin/syncthingtray" qt-widgets-gui --single-instance
Comment=Tray application for Syncthing
Icon=syncthingtray
Type=Application
Terminal=false
X-GNOME-Autostart-Delay=0
X-GNOME-Autostart-enabled=true

[Martchus]

I've just did the changes I requested in the review yesterday as it is just a 5 minutes task for me anyways. You can checkout the commits I've pushed to your branch if you're interested in the concrete changes.

[doronbehar]

Thanks!