~vcs-imports/libiconv/trunk

Don't export symbols from static MSVC .obj files.

Suggested by Dmitry Bely <dmitry.bely@gmail.com> in
<https://lists.gnu.org/archive/html/bug-gnu-libiconv/2023-08/msg00002.html>.

* include/export.h: Add a copyright notice. Set LIBICONV_DLL_EXPORTED to
empty on MSVC when DLL_EXPORT is not defined.
* Makefile.devel (include/iconv.h.build.in): Filter out the second
copyright notice.
* configure.ac (DLL_VARIABLE): Test DLL_EXPORT, not _DLL.
* lib/relocatable.h: From gnulib:
(RELOCATABLE_DLL_EXPORTED): Don't use __declspec(dllexport) when
creating static .obj files with MSVC.
* libcharset/include/export.h: Add a copyright notice. Set
LIBCHARSET_DLL_EXPORTED to empty on MSVC when DLL_EXPORT is not defined.
* libcharset/Makefile.devel (include/libcharset.h.build.in,
include/localcharset.h.build.in): Filter out the second copyright
notice.