Use GNU libunistring and Gnulib modules needed by R6RS bytevectors and ports.

* m4/gnulib-cache.m4 (gl_MODULES): Add `byteswap', `iconv_open-utf',
  `libunistring', `striconveh', and `string'.

lib/unitypes.h

@@ -0,0 +1,26 @@
+/* Elementary types for the GNU UniString library.
+   Copyright (C) 2002, 2005-2006 Free Software Foundation, Inc.
+
+   This program 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 3 of the License, or
+   (at your option) any later version.
+
+   This program 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 <http://www.gnu.org/licenses/>.  */
+
+#ifndef _UNITYPES_H
+#define _UNITYPES_H
+
+/* Get uint8_t, uint16_t, uint32_t.  */
+#include <stdint.h>
+
+/* Type representing a Unicode character.  */
+typedef uint32_t ucs4_t;
+
+#endif /* _UNITYPES_H */