Set dll exports

[MichaelChirico]

R-exts: https://cran.r-project.org/doc/manuals/r-devel/R-exts.html#Creating-shared-objects-1

Follows r-lib/xml2#473 and Rdatatable/data.table#7607.

As noted there, it's not clear this is really needed, but it does follow the practice of other packages like {Matrix}, and signals clear intent.

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I got here looking around at r-lib repos with $(C_VISIBILITY) declarations; I would add the equivalent file to all those repos if we think it's not a waste of time :)

https://github.com/search?q=org%3Ar-lib+%2F%5B%24%5D%5B%28%5DC_VISIBILITY%2F&type=code

[lionel-]

So the presence of the .def file allows proper visibility handling (e.g. hidden by default except for explicit exports) without setting the visibility flags?

[MichaelChirico]

Per Ivan's comment, it seems the visibility flags don't WAI on Windows?

Rdatatable/data.table#7607 (comment)

I also don't have a Windows machine to test, but if you have one, examine objdump.exe on the rlang.dll, my inference is it will show many objects, whereas the intent of the code as written was to only show one.