configure wchar size check

[Miroslav Lichvar]

Just a minor issue in the configure script. It uses this code to get
the size of wchar_t:

fprintf (f, "%d", (sizeof (wchar_t)));

which produces the following warning:

format '%d' expects argument of type 'int', but argument 3 has type
'long unsigned int'

I suspect it might not work as expected a 64-bit big-endian arch.
I'm not sure where that code is coming from. If I run autoconf here,
it generates a different check.

-- 
Miroslav Lichvar