Fix for installation with homebrewed Qt on OS X
(Thanks to Michka Popoff)

Change-Id: Ic77ceed0e9dc569e3998cf97b90fc18f2068d0c5
Reviewed-on: https://iffwww.iff.kfa-juelich.de/review/31
Reviewed-by: Josef Heinen <j.heinen@fz-juelich.de>