utf8.[ch] et al: use char32_t and char16_t instead of int, int32_t, int16_t

rework C11 utf8.[ch] to use char32_t instead of uint32_t when referring
to unicode chars, to make things more expressive.

[
 @zonque:
  * rebased to current master
  * use AC_CHECK_DECLS to detect availibility of char{16,32}_t
  * make utf8_encoded_to_unichar() return int
]

src/basic/utf8.h

@@ -24,12 +24,14 @@
 #include <stdbool.h>
 #include <stddef.h>
 #include <stdint.h>
+#include <uchar.h>
 
 #include "macro.h"
+#include "missing.h"
 
 #define UTF8_REPLACEMENT_CHARACTER "\xef\xbf\xbd"
 
-bool unichar_is_valid(uint32_t c);
+bool unichar_is_valid(char32_t c);
 
 const char *utf8_is_valid(const char *s) _pure_;
 char *ascii_is_valid(const char *s) _pure_;
@@ -40,20 +42,20 @@ bool utf8_is_printable_newline(const char* str, size_t length, bool newline) _pu
 char *utf8_escape_invalid(const char *s);
 char *utf8_escape_non_printable(const char *str);
 
-size_t utf8_encode_unichar(char *out_utf8, uint32_t g);
+size_t utf8_encode_unichar(char *out_utf8, char32_t g);
 char *utf16_to_utf8(const void *s, size_t length);
 
 int utf8_encoded_valid_unichar(const char *str);
-int utf8_encoded_to_unichar(const char *str);
+int utf8_encoded_to_unichar(const char *str, char32_t *ret_unichar);
 
-static inline bool utf16_is_surrogate(uint16_t c) {
+static inline bool utf16_is_surrogate(char16_t c) {
         return (0xd800 <= c && c <= 0xdfff);
 }
 
-static inline bool utf16_is_trailing_surrogate(uint16_t c) {
+static inline bool utf16_is_trailing_surrogate(char16_t c) {
         return (0xdc00 <= c && c <= 0xdfff);
 }
 
-static inline uint32_t utf16_surrogate_pair_to_unichar(uint16_t lead, uint16_t trail) {
+static inline char32_t utf16_surrogate_pair_to_unichar(char16_t lead, char16_t trail) {
                 return ((lead - 0xd800) << 10) + (trail - 0xdc00) + 0x10000;
 }