build: Search for slice directories in share without slice suffix

Some systems don't use share/Ice/slice but share/Ice directly,
e.g. the devel/ice on FreeBSD and possibly also MacPorts.

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Testing: All jobs should be green including http://ci.openmicroscopy.org/view/Failing/job/OMERO-5.1-merge-cpp/label=perch/ which is currently failing and should be fixed by this change.