perl: Enable threading in host-perl as well if threading is selected

perl: Enable threading in host-perl as well if threading is selected

While host-perl only exists to run build scripts(and thus, should have
no influence on built packages at all), most packages depend heavily on the
feature set of the perl installation that ran their configuration scripts.
This change makes them see that threading support is enabled(if it actually
is selected).

We can't use configuration symbol overrides for this one, as they are
visible to host-perl as well. Using overrides would make it act as if it
had threading support enabled without actually having it.

Signed-off-by: Marcel Denia <naoir@gmx.net>