Bug#846919: yade: enable parallel builds

[Anton Gladky]

Hi Emilio,

the only reason, why I disabled parallel builds for this package
is the frequent build failures due to insufficient RAM on build
servers.

This program requires around 2GB Ram/Thread because of
templates, Boost and meta-programming. I can try to reenable
parallel builds just to check, whether current configuration of
build-servers is suitable for that.

Best regards

Anton