Martin Sebor paraphrased:
Cygwin really doesn't define wcsnxfrm() (i.e., both
_RWSTD_NO_WCSXFRM and _RWSTD_NO_WCSXFRM_IN_LIBC are #defined)
we should probably implement our own transformation.
collate.cpp, though, I don't see a check for _RWSTD_NO_WCSXFRM_IN_LIBC,
just for the former (which only determines whether function is declared;
the latter tells us if it's also not defined).