Install libraries in correct directories on multiarch systems.

NB: this patch may change the default install dirs on some systems,
and will _not_ overwrite previous installations. Thus make sure to
link to the correct library versions of e.g. libgromacs.

Fixes #591 and #623.

Change-Id: Ibd1d7d216236a0f5841ba9f0ac622a3f8f1a8718