Support shared exodus library on windows

[gsjaardema]

Patch from external to support building shared exodus library on windows for use with exodus.py.

HI Greg,

I got an initial build with exodus3.py working on Windows.
Attached is the patch to make that work.

Many of the errors I was getting before were related to building Seacas against a shared exodus library. Once I changed TRIBITS_ADD_LIBRARY to ADD_LIBRARY, some confusion about which libraries to link with was cleared up.
I see that for building a static library, ADD_LIBRARY was already used.

In exodusII.h, I added a check for defining __declspec(dllexport) as needed by exodus3.py.

I think long term, if you needed the dllimport, and if you needed to support both shared and static builds of exodus at the same time, I think one could add a PUBLIC COMPILE_DEFINITIONS property on exodus_shared to indicate the exodusII.h will be used with a shared library. That would allow exodusII.h to be used for both static and shared builds. Putting static vs. shared information into a generated header file might prevent using the headers for both static and shared libraries. The current addition of __declspec(dllexport) is compatible with the potential long term change proposed here.

Thanks,
Clint