[bug-gnu-libiconv] Avoid __declspec(dllexport) for MSVC static build

[Dmitry Bely]

libiconv always defines BUILDING_DLL for its build. In the case of MSVC that in turn defines

# define RELOCATABLE_DLL_EXPORTED __declspec(dllexport)

That's unacceptable for a static library because if that library is linked into a user DLL, all its public symbols are automatically exported from the DLL.

BUILDING_DLL should be defined only when shared libraries are built.

- Dmitry Bely