Add support for using utf8proc with --enable-utf8proc, useful for platforms

(like OS X) where the system implementation is crap. From Joshua Rubin.

utf8.c

@@ -109,7 +109,11 @@ utf8_width(wchar_t wc)
 {
 	int	width;
 
+#ifdef HAVE_UTF8PROC
+	width = utf8proc_wcwidth(wc);
+#else
 	width = wcwidth(wc);
+#endif
 	if (width < 0 || width > 0xff) {
 		log_debug("Unicode %04x, wcwidth() %d", wc, width);
 
@@ -135,7 +139,11 @@ utf8_width(wchar_t wc)
 enum utf8_state
 utf8_combine(const struct utf8_data *ud, wchar_t *wc)
 {
+#ifdef HAVE_UTF8PROC
+	switch (utf8proc_mbtowc(wc, ud->data, ud->size)) {
+#else
 	switch (mbtowc(wc, ud->data, ud->size)) {
+#endif
 	case -1:
 		log_debug("UTF-8 %.*s, mbtowc() %d", (int)ud->size, ud->data,
 		    errno);
@@ -155,7 +163,11 @@ utf8_split(wchar_t wc, struct utf8_data *ud)
 	char	s[MB_LEN_MAX];
 	int	slen;
 
+#ifdef HAVE_UTF8PROC
+	slen = utf8proc_wctomb(s, wc);
+#else
 	slen = wctomb(s, wc);
+#endif
 	if (slen <= 0 || slen > (int)sizeof ud->data)
 		return (UTF8_ERROR);