Annotate Fruit's exported symbols

[poletti-marco]

If Fruit is compiled normally, without using -fvisibility=hidden and then used in client code compiled with -fvisibility=hidden switch, the resulting executable segfaults (this issue was reported by Christoph Duelli).
Ideally, Fruit should support client code that uses -fvisibility=hidden.

Once -fvisibility=hidden is supported, it might be worth also using it when compiling Fruit itself, potentially reducing the compiled size.

[poletti-marco]

Now that Fruit supports MSVC, this is required to support building Fruit as a dll with MSVC.

[poletti-marco]

Now that Fruit supports MSVC, this is required to support building Fruit as a dll with MSVC.

Actually I was wrong, I've now added support for building Fruit as a dll using MSVC with CMake's WINDOWS_EXPORT_ALL_SYMBOLS. So this is no longer blocking full MSVC support.