uchar.h: define char16_t and char32_t for old c++

including uchar.h in c++ code is only well defined in c++11 onwards
where char16_t and char32_t type definitions must be hidden since they
are keywords.  however some c++ code compiled for older c++ standard
include uchar.h too and they need the typedefs, this fix makes such
code work.

include/uchar.h

@@ -3,7 +3,9 @@
 
 #ifdef __cplusplus
 extern "C" {
-#else
+#endif
+
+#if __cplusplus < 201103L
 typedef unsigned short char16_t;
 typedef unsigned char32_t;
 #endif