install host libraries into -hostprefix

... and introduce -hostlibdir configure option for symmetry.

the libraries built for the host have no business in the target prefix.

in principle this code would even support dynamically linked host
libraries, but that's currently unused.

Task-number: QTBUG-30591
Change-Id: I8e600fa4911a020fb0e87fbf7ef2f35647c7c4d5
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>
Reviewed-by: Ivan Romanov <drizt@land.ru>