Required fixes to build libzt on Windows

[jbatnozic]

I'm maintaining a homemade Conan package for libzt and I'm constantly having to apply patches to make it build properly on Windows. (note that I'm building it purely using CMake and not using the provided powershell scripts, though I doubt it would change things)

I will lay out here the recurring needed changes:

Linking declarations

[Link to relevant code]

This should be as follows:

#if defined(_WIN32)
    #ifndef ZTS_STATIC
        #ifdef ADD_EXPORTS
            #define ZTS_API __declspec(dllexport)
        #else
            #define ZTS_API __declspec(dllimport)
        #endif
    #else
        #define ZTS_API
    #endif
 	#define ZTCALL __cdecl
#else
    #define ZTS_API
    #define ZTCALL
#endif

We need these dllimport/dllexport declarations only when building a .dll, and NOT when building a static library. Currently in my Conan package I optionally provide this preprocessor define ZTS_STATIC based on the conan build configuration (static/shared). I see that the root CMakeLists.txt has multiple configurations at the same time so I wasn't sure how it would handle this - maybe per (cmake) target?

Side note: could we rename ADD_EXPORTS -> ZTS_ADD_EXPORTS? If you mixed this with another library which defined the generic ADD_EXPORTS you'd be stuck with either having both shared or both static (not always a problem but who knows?)

zts_errno linking

[Link to relevant code]

This should be as follows:

extern ZTS_API int zts_errno;

Otherwise you can't use this crucial variable at all when libzt is built as a DLL.

Side note: is this variable thread-local?

nlohmann/json include paths

[Link to relevant code]

The following line needs to be added:

include_directories(${ZTO_SRC_DIR}/ext/)

Otherwise certain headers from ZeroTierOne will try to #include <nlohmann/json.hpp> and won't be able to find it so the build will fail. I'm surprised that it even builds without this on other platforms (system package maybe?).

[janvanbouwel]

As for nlohmann/json, this is disabled on other platforms and for some reason not on Windows, found it recently as part of #63 but it seemed someone else was working on it so I suggested the fix there for now IceSoulHanxi#2.

[jbatnozic]

What about the others? I'd open a PR myself but I have no permissions on your repo.

[janvanbouwel]

No idea sorry. (I'm not associated with ZeroTier to be clear, I've been working on nodejs bindings so know some parts of the project well.)

I'd open a PR myself but I have no permissions on your repo.

I'm a bit confused by what you mean? You haven't forked the repo, how are you trying to open a PR?

[jbatnozic]

Apparently it's me who was confused here. So far I hadn't used GitHub to contribute to projects which weren't my own, so I wasn't aware that I had to fork the repo first. The whole time I was trying to set up my own branch in this repo.