cmake windows shared builds do not export symbols

When building shared libs on windows, using cmake and `-DBUILD_SHARED_LIBS=On`, no .lib file is produced, which in turn doesn't allow a consumer to link against cctz.

Ideally, symbols should be properly exported (see https://msdn.microsoft.com/en-us/library/a90k134d.aspx), then a lib file will be generated.

A not-so-elegant, but working solution may be to enable: `CMAKE_WINDOWS_EXPORT_ALL_SYMBOLS`, which I had success with, however for optimal results you should only export what should be exported.

This workaround is as simple as adding:

`set(CMAKE_WINDOWS_EXPORT_ALL_SYMBOLS YES CACHE BOOL "Export all symbols")`

or running the cmake config command with `-DCMAKE_WINDOWS_EXPORT_ALL_SYMBOLS=True`

thanks.