Expansion of @libexecdir@ in .desktop.in file includes ${exec_prefix}

[Stefan Koch <stefan.koch10@xxxxxxxxx>]

Hi

The line:
Exec=@libexecdir@/usbauth-notifier/usbauth-notifier

from 
https://github.com/kochstefan/usbauth-all/blob/master/usbauth-notifier/data/usbauth-notifier.desktop.in

will expanded to:
Exec=${exec_prefix}/libexec/usbauth-notifier/usbauth-notifier

But desktop-Files doesn't allow variables.

Do you have an idea how to get the line expanded to:
Exec=/usr/libexec/usbauth-notifier/usbauth-notifier

without the ${exec_prefix} variable?

Or is there a way to get only "libexec" string from a @variable@ macro?
On SUSE @libexecdir@ must expanded to /usr/lib

Thanks and best regards

Stefan