Fix a misnomer.

Fix a misnomer.

* include/export.h (LIBICONV_SHLIB_EXPORTED): Renamed from
LIBICONV_DLL_EXPORTED. Prefer the term "shared library", since the term
"DLL" applies only to Windows.
* Makefile.devel (include/iconv.h.build.in): Insert
LIBICONV_SHLIB_EXPORTED instead of LIBICONV_DLL_EXPORTED.
* configure.ac (DLL_VARIABLE): Update comment.
* lib/Makefile.in (DEFS): Update comment.
* woe32dll/export.h: Update comment.
* libcharset/include/export.h (LIBCHARSET_SHLIB_EXPORTED): Renamed from
LIBCHARSET_DLL_EXPORTED. Prefer the term "shared library", since the
term "DLL" applies only to Windows.
* libcharset/Makefile.devel (include/libcharset.h.build.in,
include/localcharset.h.build.in): Insert LIBCHARSET_SHLIB_EXPORTED
instead of LIBCHARSET_DLL_EXPORTED.
* libcharset/lib/relocatable-stub.c (libcharset_set_relocation_prefix): Use
LIBCHARSET_SHLIB_EXPORTED instead of LIBCHARSET_DLL_EXPORTED.
* libcharset/lib/Makefile.in (DEFS): Update comment.
* libcharset/Makefile.in (install-lib): Update comment.