define __STDC_UTF_{16,32}__ macros

these macros are used to indicate that the implementation uses,
respectively, utf-16 and utf-32 encoding for char16_t and char32_t.

include/stdc-predef.h

@@ -7,4 +7,7 @@
 #define __STDC_IEC_559__ 1
 #endif
 
+#define __STDC_UTF_16__ 1
+#define __STDC_UTF_32__ 1
+
 #endif