Unable to find dataset images when dataset is installed after ViSP is built

[fspindle]

When ViSP is build prior to download and set VISP_INPUT_IMAGE_PATH some examples and tests that are using dataset images don't find corresponding *.png images.

To reproduce the issue:

$ cd visp-build
$ make
$ cat ViSP-third-party.txt
  Tests and samples: 
    ...
    Dataset found:               no

Then install dataset following for example this tutorial.

At this point you will encounter the following issue:

$ cd example/tracking
$ ./trackDot
Load: /Users/username/visp/visp-images/mire-2/image.0001.pgm

ERROR:
  Cannot read /Users/username/visp/visp-images/mire-2/image.0001.pgm
  Check your -i /Users/username/visp/visp-images option
  or VISP_INPUT_IMAGE_PATH environment variable.