Remove reference to openni-dev in pkg-config files.

OpenNI packages are not currently shipped with any pkg-config
files. Furthermore, Debian packages libopenni-dev and openni-dev are
currently not coinstallable and provide pkg-config files with
different namings. To workaround these issues, this patch disables
OpenNI check in pkg-config files.

This partially solves issue #221.
CC @thomas-moulard