Static data members in LIBZIM_API and/or LIBZIM_PRIVATE_API classes result in linker errors under Windows CI

[veloman-yunkan]

Examples:

• https://github.com/openzim/libzim/actions/runs/14819235224/job/41603865419?pr=984
• 3dd1a5b

The problem seems to be caused by the improper definition of the LIBZIM_API and LIBZIM_PRIVATE_API macros.

1. When building a DLL they need to be defined as __declspec(dllexport)
2. However when compiling client code to be linked with a DLL they must be set to __declspec(dllimport)

See https://learn.microsoft.com/en-us/cpp/build/importing-into-an-application-using-declspec-dllimport?view=msvc-170

[veloman-yunkan]

This concern was raised during the review of #783 and the suggestion was to address it in a follow-up PR which was not done.

[kelson42]

@veloman-yunkan I have difficulties to assess the level of priority. Any help welcome here about urgency and level of impact.

[veloman-yunkan]

It is not urgent. I worked around the issue another time in #984 but it would be nice if we didn't have to deal with it every now and then.