Windows DLL support

[marcstern]

Current code is incompatible with Windows DLL.
Solution is simple. In .H files:

#ifdef WIN32
# ifdef CJOSE_BUILD
#  define CJOSE_EXTERN __declspec(dllexport)
# else
#  define CJOSE_EXTERN __declspec(dllimport)
# endif
#else
# define CJOSE_EXTERN
#endif
[...]
CJOSE_EXTERN const char *CJOSE_HDR_ALG;

Example is for CJOSE_HDR_ALG, but should be added to all exported symbols.
The #ifdef WIN32 has to be included once in a central file obviously.

Could we expect this?
Thanks a lot

[zandbelt]

since the Windows build requires other Windows-specific files as well I suggest to keep this in a fork

[marcstern]

I build it in Windows with only that addition