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/geniconv/geniconv.h

@@ -0,0 +1,59 @@
+/*
+  Universal iconv implementation for OS/2.
+
+  Andrey Vasilkin, 2016.
+*/
+
+#ifndef GENICONV_H
+#define GENICONV_H
+
+#include <iconv.h>
+
+#ifdef iconv_open
+#undef iconv_open
+#endif
+#define iconv_open libiconv_open
+
+#ifdef iconv
+#undef iconv
+#endif
+#define iconv libiconv
+
+#ifdef iconv_close
+#undef iconv_close
+#endif
+#define iconv_close libiconv_close
+
+#define iconv_clean libiconv_clean
+
+// Non-standard function for iconv to unload the used dynamic library.
+void libiconv_clean();
+
+iconv_t libiconv_open(const char* tocode, const char* fromcode);
+size_t libiconv(iconv_t cd, char* * inbuf, size_t *inbytesleft,
+                 char* * outbuf, size_t *outbytesleft);
+int libiconv_close(iconv_t cd);
+
+// System codepage <-> UTF-8.
+
+// StrUTF8()
+// Coverts string from system cp. to UTF-8 (fToUTF8 is not 0) or from UTF-8 to
+// the system cp. (fToUTF8 is 0). Converted ASCIIZ string will be placed at the
+// buffer pcDst, up to cbDst - 1 (for sys->utf8) or 2 (for utf8->sys) bytes.
+// Returns the number of bytes written into pcDst, not counting the terminating
+// 0 byte(s) or -1 on error.
+int StrUTF8(int fToUTF8, char *pcDst, int cbDst, char *pcSrc, int cbSrc);
+
+// StrUTF8New()
+// Coverts string from system cp. to UTF-8 (fToUTF8 is not 0) or from UTF-8 to
+// the system cp. (fToUTF8 is 0). Memory for the new string is obtained by
+// using libc malloc().
+// Returns converted string, terminating two bytes 0 is appended to the result.
+// Returns null on error.
+char *StrUTF8New(int fToUTF8, char *pcStr, int cbStr);
+
+// StrUTF8Free()
+// Deallocates the memory block located by StrUTF8New() (just libc free()).
+void StrUTF8Free(char *pszStr);
+
+#endif // GENICONV_H