The target name for the botan lib depends on release or debug mode,
so define it after forcing release.

Resulted in link error on Mac/Win, where botan was in (lib)Botan(_debug|d) but
was linked against as (lib)Botan.

Change-Id: I7bf27526065ca740a97d2379af8424ab71274e96
Reviewed-by: Joerg Bornemann <joerg.bornemann@nokia.com>