[libstdc++] Avoid referencing wchar/mbchar functions when not provided by C library.

[Gabriel Dos Reis]

Bernardo Innocenti <bernie@develer.com> writes:

| 2003-08-09  Bernardo Innocenti  <bernie@develer.com>
| 
| 	* include/c_std/std_cstdlib.h: Avoid using missing C library symbols.

OK.

-- Gaby