And guard <cuchar> too, why not... closes hsutter#16

I was trying to rely on only the named feature tests, but if adding this `__has_include` can unblock someone then let's do it. Also add a `(void)` cast to `~c_raii()` to silence a Clang warning

Author: hsutter

include/cpp2util.h

@@ -106,7 +106,9 @@
  #include <cctype>
  #include <charconv>
  #include <cstring>
- #include <cuchar>
+ #if __has_include(<cuchar>)
+ #include <cuchar>
+ #endif
  #include <cwchar>
  #include <cwctype>
  #ifdef __cpp_lib_format
@@ -777,7 +779,7 @@ class c_raii {
  template<typename D>
  c_raii( T t_, D d )
  : t{ t_ }
- , dtor{ [](void* x) { (D)(x); } }
+ , dtor{ [](void* x) { (void)(D)(x); } }
  { }
 
  ~c_raii() { if (dtor) dtor(t); }