Instead of checking whether the library path ends up somewhere in /usr,
set the architecture prefix, anyway. The library path mechanism is also
used in custom prefixes, without this change, the library gets installed
globally on Debian and Ubuntu, leading to non-standard behaviour and
cross-build problems, as multiple architecture builds of this library
can't be installed alongside each other.
This is the minimal change to correct this behaviour from upstream cmake.
Sebastian Kügler