eliminate bits/wchar.h

the preprocessor can reliably determine the signedness of wchar_t.
L'\0' is used for 0 in the expressions so that, if the underlying type
of wchar_t is long rather than int, the promoted type of the
expression will match the type of wchar_t.

include/wchar.h

@@ -24,7 +24,13 @@ extern "C" {
 
 #include <bits/alltypes.h>
 
-#include <bits/wchar.h>
+#if L'\0'-1 > 0
+#define WCHAR_MAX (0xffffffffu+L'\0')
+#define WCHAR_MIN (0+L'\0')
+#else
+#define WCHAR_MAX (0x7fffffff+L'\0')
+#define WCHAR_MIN (-1-0x7fffffff+L'\0')
+#endif
 
 #define NULL 0L