gunicollate/cygwin: Don't use __STDC_ISO_10646__ for wchar_t related checks
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.
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