The code in gunicollate uses __STDC_ISO_10646__ to check that wchar.h is avilable,
that it includes the wide character related functions and that sizeof(wchar_t) == 4.

cygwin defines __STDC_ISO_10646__ and has sizeof(wchar_t) == 2 and the C standard text isn't
that clear on whether wchar_t should always be 4 bytes in this case, so we better not use if for
assuming the size here.

Instead of relying on __STDC_ISO_10646__ add HAVE_WCHAR_H and SIZEOF_WCHAR_T macros.
With HAVE_WCHAR_H defined we assume wchar_t exists and wchar.h exists. With SIZEOF_WCHAR_T we
guard the parts where the size of wchar_t is assumed to be 4 (currently all of them).

Note that this doesn't make the collate tests pass under cygwin, they fail before and after this patch for me.

See !755 for related discussions.