[CMake] fix StandaloneTools build conditions

Lea Vauchierrequested to merge
bugfix/fixStandaloneToolsBuildConditions into master
  • add nanoflann_FOUND condition to build PostProcessLabelCloud
  • remove dependency of LabelCloudFromImagesDetections on nanoflann

@melanie.carriere @nick.laurenson @nicolas.cadart please review.

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