mtools: ensure install-info isn't used

mtools calls install-info at 'make install' time if available after
installing the mtools info page - But as we don't use the info pages for
anything / remove in target-finalize, this is a waste of time and a
potential cause of build failures as reported on the list:

http://lists.busybox.net/pipermail/buildroot/2016-May/160604.html



So ensure configure doesn't find it.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>