Fix Windows paths error found by Javier Colmeiro (rebased onto develop)



This is the same as gh-698 but rebased onto develop.

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Compare the last bullet point of https://www.openmicroscopy.org/site/support/omero5/developers/Cpp.html#preparing-to-build to https://www.openmicroscopy.org/site/support/omero5-staging/developers/Cpp.html#preparing-to-build