Windows: properly support shared library build

[mwestphal]

It looks like BUILD_SHARED_LIBS=ON was not considered on Windows before, this fixes it by:

• Installing the DLL using RUNTIME DESTINATION
• Exporting all the symbols (which really should be done symbol by symbol in the lib)

I agree that my contributions are licensed under the MIT License.
You can find a copy of this license at https://opensource.org/licenses/MIT

[dacap]

Thanks again @mwestphal! I'll merge this as it is. I have plans to include a CLIP_DLL macro to import/export functions/classes in a near future.

[mwestphal]

Thanks for the merge! Do you plan to do a release soon ? :)

[dacap]

:) I'll try to tag a release today, probably adding explicit exports/imports and adding the shared version to github workflows.