OpenNI: allow to override XN_OPEN_NI_FILES_LOCATION
Include: fix header install rules (install arch dependent header)
OpenNI: restore the original value to XN_OPEN_NI_FILES_LOCATION (ie: /var/lib/ni)

From now, any override of this location should be done passing
"-DXN_OPEN_NI_FILES_LOCATION=..." on the cmake commande line
(especially in the Aldebaran case, which is /usr/share/ni).