Add dllexport/dllimport to make it easier to use Sonic as a DLL

[gexgd0419]

There's no __declspec(dllexport) or __declspec(dllimport) in the header file, so Sonic cannot be used directly as a DLL without modification, because nothing would be exported. Compiling a Windows DLL requires functions to be explicitly exported.

A .def file can be added to export the functions as a workaround. However, the same header file is usually used when importing the functions as well. Without __declspec(dllimport), the compiler will assume that it is a regular extern function, and some optimizations for imported functions will not be applied.

To support compiling to both DLL and static library, #ifdef can be used to check a specific macro, for example SONIC_DLL. If not defined, __declspec(dllexport) and __declspec(dllimport) will not be added to keep the original behavior. Those who want to use Sonic as a DLL would have to predefine this macro.

There's a related pull request #27 but it hasn't been merged or closed for a long time. So @waywardgeek what's your thought about that?