i#4654: Require doxygen for test suite builds (#4655)
i#4654: Require doxygen for test suite builds (#4655)

Makes doxygen required for all test suite builds, including Windows,
rather than just a soft warning.  We need it to work in order to
create release packages, after all.

Fixes #4654
AmesianX/dynamorio,AmesianX/dynamorio,AmesianX/dynamorio,AmesianX/dynamorio,AmesianX/dynamorio