CMake: Fixes for fatal warnings and doxygen

- Add `-Dfatal-warnings=ON|OFF` option.  This enables `-Wextra` with GCC/clang and `/WX` for MSVC and checks the flag is supported as for all other options.
- Minor doxygen fix.

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Testing:

Run cmake with `fatal-warnings` set to ON or OFF.  The build should fail when set to ON (since it's not free of warnings).  This should also apply to the OmeroCpp build with the submodule updated.