C++: Correct version embedding

The UNIX commit date wasn't being embedded or used properly.  Update the ant and source-version code to embed properly, and the cmake code to use it properly.

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Testing:

- Run `ant release`
- Run cmake against the unzipped source tree
- The cmake output should include the correct release version, plus the git hash and commit date
- The C++ build should then proceed without errors (`cpp/lib/ome/internal/version.h` should contain the correct data for OME_VCS_REVISION, OME_VCS_DATE and OME_VCS_DATE_S.

Will fix: https://ci.openmicroscopy.org/job/BIOFORMATS-5.1-latest-cpp/ which was updated to test source building.