Force 64-bit build to avoid compatibility issues
(10.6.8 build not working on Yosemite)

Change-Id: I731e4644171e8a3c93b4f84547cfd6ab93477508
Reviewed-on: https://iffwww.iff.kfa-juelich.de/review/133
Reviewed-by: Josef Heinen <j.heinen@fz-juelich.de>