Cross-configuration changes: make $from to rm the files

Cross-configuration changes: make $from to rm the files
before copying them over (this means that he rm can be
removed from the fflushall test), create $targetmkdir
(not documented in install since it is supposed to internal
use only), make $run to copy over the executable only if
a stamp file .xok doesn't already exist (to avoid unnecessary
copying of the executable)

p4raw-id: //depot/perl@10788