Export symbols from C library when compiling as a shared library on Windows

[paleolimbot]

We will have to check that this is actually the problem, but it may be that the lack of any exported symbols causes a problem when creating a shared library on Windows ( mesonbuild/wrapdb#1536 (comment) ). adbc.h has:

// Storage class macros for Windows
// Allow overriding/aliasing with application-defined macros
#if !defined(ADBC_EXPORT)
#if defined(_WIN32)
#if defined(ADBC_EXPORTING)
#define ADBC_EXPORT __declspec(dllexport)
#else
#define ADBC_EXPORT __declspec(dllimport)
#endif  // defined(ADBC_EXPORTING)
#else
#define ADBC_EXPORT
#endif  // defined(_WIN32)
#endif  // !defined(ADBC_EXPORT)

https://github.com/apache/arrow-adbc/blob/main/adbc.h#L152-L164

(Although I think this is slightly incorrect in that _WIN32 should maybe be _MSC_VER to better support gcc on Windows via msys2).

[lidavidm]

(Although I think this is slightly incorrect in that _WIN32 should maybe be _MSC_VER to better support gcc on Windows via msys2).

Ah, probably. I derived this from some (by now old) copy of the Arrow-C++ equivalents.

[WillAyd]

(Although I think this is slightly incorrect in that _WIN32 should maybe be _MSC_VER to better support gcc on Windows via msys2).

May also be worth checking for __CYGWIN__

https://stackoverflow.com/a/9026009/621736

[WillAyd]

Also https://www.msys2.org/wiki/Porting/#platform-checks

[WillAyd]

Not to try and expand the scope too much but this gcc article might even help get these to work with GNU compilers:

https://gcc.gnu.org/wiki/Visibility