Externals: Update libiconv to 1.18

I also excluded libiconv from the Windows CMake builds since it's never used there

Externals/libiconv/libcharset/include/export.h

@@ -0,0 +1,36 @@
+/* Control of exported symbols from libcharset.
+   Copyright (C) 2005-2023 Free Software Foundation, Inc.
+
+   This file is free software: you can redistribute it and/or modify
+   it under the terms of the GNU Lesser General Public License as
+   published by the Free Software Foundation; either version 2.1 of the
+   License, or (at your option) any later version.
+
+   This file is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public License
+   along with this program.  If not, see <https://www.gnu.org/licenses/>.  */
+
+#if @HAVE_VISIBILITY@ && BUILDING_LIBCHARSET
+# define LIBCHARSET_SHLIB_EXPORTED __attribute__((__visibility__("default")))
+#elif defined _MSC_VER && BUILDING_LIBCHARSET
+/* When building with MSVC, exporting a symbol means that the object file
+   contains a "linker directive" of the form /EXPORT:symbol.  This can be
+   inspected through the "objdump -s --section=.drectve FILE" or
+   "dumpbin /directives FILE" commands.
+   The symbols from this file should be exported if and only if the object
+   file gets included in a DLL.  Libtool, on Windows platforms, defines
+   the C macro DLL_EXPORT (together with PIC) when compiling for a shared
+   library (called DLL under Windows) and does not define it when compiling
+   an object file meant to be linked statically into some executable.  */
+# if defined DLL_EXPORT
+#  define LIBCHARSET_SHLIB_EXPORTED __declspec(dllexport)
+# else
+#  define LIBCHARSET_SHLIB_EXPORTED
+# endif
+#else
+# define LIBCHARSET_SHLIB_EXPORTED
+#endif