COMP: host platform source dir check

Fix issue ITK-3164: When cross-compiling
from a linux host to a windows target,
the source directory length check should
check for the host platform instead of the
target platform.

Change-Id: I0825f201cc0b577b3c51ba3d6f71b214ef60d2b0
Merge topic 'host-directory-length-check'

d665188 COMP: host platform source dir check