This is not about a function, but about the variable _gpgrt_functions_w32_pollable. And this is not about exporting the variable from the library, but about declaring it as extern in gpgrt-int.h, so that gpgrt-int.h can be included in multiple translation units without defining the variable in each.

C++2a will have a <version> header, so some trunk libc++ headers now (indirectly) #include <version>, and on a case-insensitive file-system, when compiling a gpgme source file with "unlucky" -I../../.. switches against such trunk libc++, that can mean that such an #include <version> picks up gpgme's VERSION file.