libgpg-error-1.35.tar.bz2's libgpg-error-1.35/src/gpgrt-int.h contains
#if HAVE_W32_SYSTEM
/* Prototypes for w32-estream.c. */
struct cookie_io_functions_s _gpgrt_functions_w32_pollable;
int _gpgrt_w32_pollable_create (void *_GPGRT__RESTRICT *_GPGRT__RESTRICT cookie,
unsigned int modeflags,
struct cookie_io_functions_s next_functions,
void *next_cookie);
int _gpgrt_w32_poll (gpgrt_poll_t *fds, size_t nfds, int timeout);
#endif /*HAVE_W32_SYSTEM*/Not sure how MSVC manages to not make this an ODR violation, where _gpgrt_functions_w32_pollable is defined in every translation unit including gpgrt-int.h. But compiling with clang-cl (which I happen to do when libgpg-error is built as part of building LibreOffice; the libgpg-error version used there is 1.27, but the issue appears to be still the same with latest 1.35) fails with
libgpg_error_la-visibility.obj : warning LNK4006: _gpgrt_functions_w32_pollable already defined in libgpg_error_la-b64dec.obj; second definition ignored libgpg_error_la-estream-printf.obj : warning LNK4006: _gpgrt_functions_w32_pollable already defined in libgpg_error_la-b64dec.obj; second definition ignored libgpg_error_la-estream.obj : warning LNK4006: _gpgrt_functions_w32_pollable already defined in libgpg_error_la-b64dec.obj; second definition ignored libgpg_error_la-init.obj : warning LNK4006: _gpgrt_functions_w32_pollable already defined in libgpg_error_la-b64dec.obj; second definition ignored libgpg_error_la-w32-estream.obj : warning LNK4006: _gpgrt_functions_w32_pollable already defined in libgpg_error_la-b64dec.obj; second definition ignored libgpg_error_la-w32-iconv.obj : warning LNK4006: _gpgrt_functions_w32_pollable already defined in libgpg_error_la-b64dec.obj; second definition ignored libtool: link: ( cd ".libs" && rm -f "libgpg-error.la" && ln -s "../libgpg-error.la" "libgpg-error.la" ) libtool: link: C:/lo-clang/core/workdir/LinkTarget/Executable/gcc-wrapper.exe -g -o gen-w32-lock-obj.exe gen-w32-lock-obj.obj libtool: link: C:/lo-clang/core/workdir/LinkTarget/Executable/gcc-wrapper.exe -g -o gpg-error.exe gpg_error-strsource-sym.obj gpg_error-strerror-sym.obj gpg_error-gpg-error.obj ./.libs/libgpg-error.lib libgpg-error.lib(libgpg_error_la-init.obj) : error LNK2005: _gpgrt_functions_w32_pollable already defined in libgpg-error.lib(libgpg_error_la-visibility.obj) libgpg-error.lib(libgpg_error_la-estream.obj) : error LNK2005: _gpgrt_functions_w32_pollable already defined in libgpg-error.lib(libgpg_error_la-visibility.obj) libgpg-error.lib(libgpg_error_la-estream-printf.obj) : error LNK2005: _gpgrt_functions_w32_pollable already defined in libgpg-error.lib(libgpg_error_la-visibility.obj) libgpg-error.lib(libgpg_error_la-b64dec.obj) : error LNK2005: _gpgrt_functions_w32_pollable already defined in libgpg-error.lib(libgpg_error_la-visibility.obj) libgpg-error.lib(libgpg_error_la-w32-estream.obj) : error LNK2005: _gpgrt_functions_w32_pollable already defined in libgpg-error.lib(libgpg_error_la-visibility.obj) gpg-error.exe : fatal error LNK1169: one or more multiply defined symbols found clang: error: linker command failed with exit code 1169 (use -v to see invocation)
Adding "extern" to the declaration of _gpgrt_functions_w32_pollable in src/gpgrt-int.h fixes that.