cpp: Windows fixes for embedded gtest

This makes gtest linking work on Windows.  Check OMERO-5.1-merge-cpp-win; you'll see it fail but the failures will no longer include gtest link errors.

There may be better solutions to the problem than this, but this gets it working and we can see what works better (if anything) later.

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Testing: Check the latest logs in http://ci.openmicroscopy.org/job/OMERO-5.1-merge-cpp-win/ and compare with yesterday's.