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The pragma to ignore 4251 already existed in windowsIncludes.h, and it works
when compiling lepton itself. However, when clients use lepton, they get this
warning. I think this is why @sherm1 put the define as shown here:
https://github.com/simbody/simbody/blob/master/SimTKcommon/include/SimTKcommon/internal/common.h#L168
 (whether or not the shared library is being built or used).

Clang had been generating the following warning:

```
ParsedExpression.cpp:123:13:
warning:
      30 enumeration values not handled in switch: 'CONSTANT', 'VARIABLE',
      'CUSTOM'...
            [-Wswitch]
                switch (node.getOperation().getId()) {
                                ^
                                1 warning generated.
```

To get rid of this warning, I added a default case, which does nothing.