Merged in david_s5/nuttx/upstream_wc (pull request #456)

cwchar:Use CONFIG_LIBC_WCHAR to only export the wc/mb functions

Approved-by: Gregory Nutt <gnutt@nuttx.org>

include/cxx/cwchar

@@ -54,6 +54,7 @@ namespace std
   using ::wctype_t;
   using ::size_t;
 
+#ifdef CONFIG_LIBC_WCHAR
   using ::btowc;
   using ::fwprintf;
   using ::fwscanf;
@@ -132,6 +133,7 @@ namespace std
   using ::wmemset;
   using ::wprintf;
   using ::wscanf;
+#endif
 };
 
 #endif // __INCLUDE_CXX_CWCHAR