configure: fail on missing gperf

Previously GPERF was set to 'libetonyek/missing --run gperf', which
doesn't make too much sense. Better to fail during configure instead of
during make if gperf is missing.

Change-Id: I144820f6202d27e5e4d6a45247a0c48b9611a041
Reviewed-on: https://gerrit.libreoffice.org/6596
Reviewed-by: David Tardon <dtardon@redhat.com>
Tested-by: David Tardon <dtardon@redhat.com>