support passing libperl to configure

Configure allows to pass a -Dlibperl=libperl.so.SOMETHING parameter.
make sure that perlcross handles that too
dynaloader is needed to build Makefiles

I get parallel make failures without this.
While it does introduce a circular dynaloader <--> dynaloader dependency,
make hust drops that when it encounters it.
set a default vendorprefix

this makes the vendor* settings behave like the site* and core* ones if
usevendorprefix is defined.
makes it easier to actually use the vendor stuff without having to
define everything manually.
there is no such thing as {site,vendor}scriptdir