How to handle static variables in shared libraries in Windows

[claudiofantacci]

I noticed that we did not mention how to handle static variables in shared libraries in Windows. Unfortunately those are not handled by CMAKE_WINDOWS_EXPORT_ALL_SYMBOLS. While the blog post that we link in the README mention that, I wonder if we should stress this more, or we can simply ignore this relatively niche use case. See ms-iot/ROSOnWindows#33 and ms-iot/ROSOnWindows#37 for a related discussion.

Originally posted by @traversaro in #36 (comment)