Many C4275 warning compiling on Windows

[AWhetter]

When compiling the shared libraries of IlmBase on Windows, there are many print outs of the warning

IlmBase\Iex\IexBaseExc.h(143) : warning C4275: non dll-interface class 'Iex::BaseExc' used as base for dll-interface class 'Iex::ArgExc'
        IlmBase\Iex\IexBaseExc.h(60) : see declaration of 'Iex::BaseExc'
        IlmBase\Iex\IexBaseExc.h(143) : see declaration of 'Iex::ArgExc'

This also occurs for every other exception in IexBaseExc.h at inherits from Iex::BaseExc.

When compiling external projects that use Iex, the error shows there as well.

[meshula]

The problem stems from the fact that Iex::BaseExc is subclassed from std::string which does not have a DLL adornment. Although it is easy to refactor Iex::BaseEXc so that it does not derive from string, that is unfortunately a breaking change for some applications.

[cary-ilm]

Looking into the OpenEXR issue backlog. This was addressed in v2.3.0, Iex::BaseExc is no longer derived from std::string.