os2: add port files for SDL2-2.0.4 from Andrey Vasilkin

only geniconv/iconv.h (was from LGPL libiconv) is replaced with a generic
minimal iconv.h based on public knowledge.

src/core/os2/SDL_os2.c

@@ -0,0 +1,36 @@
+/*
+  Simple DirectMedia Layer
+  Copyright (C) 1997-2016 Sam Lantinga <slouken@libsdl.org>
+
+  This software is provided 'as-is', without any express or implied
+  warranty.  In no event will the authors be held liable for any damages
+  arising from the use of this software.
+
+  Permission is granted to anyone to use this software for any purpose,
+  including commercial applications, and to alter it and redistribute it
+  freely, subject to the following restrictions:
+
+  1. The origin of this software must not be misrepresented; you must not
+     claim that you wrote the original software. If you use this software
+     in a product, an acknowledgment in the product documentation would be
+     appreciated but is not required.
+  2. Altered source versions must be plainly marked as such, and must not be
+     misrepresented as being the original software.
+  3. This notice may not be removed or altered from any source distribution.
+*/
+#include "../../SDL_internal.h"
+
+#if defined(__OS2__)
+
+#include "core/os2/geniconv/geniconv.h"
+
+// SDL_OS2Quit() will be called from SDL_QuitSubSystem().
+
+void SDL_OS2Quit()
+{
+  // Unload DLL used for iconv. We can do it at any time and use iconv again -
+  // dynamic library will be loaded on first call iconv_open() (see geniconv).
+  libiconv_clean();
+}
+
+#endif