cdefs: Document what we do when _XOPEN_SOURCE is an empty string

X/Open originally had _XOPEN_SOURCE defined to signify conformance with
the Single Unix Specification, starting with its third iteration. There
it defined _XOPEN_SOURCE being defined as the same thing as
_POSIC_C_SOURCE=2, though the different versions of the spec had slight
variances as to what's defined and wheter or not _XOPEN_SOURCE_EXTENSION
needed to be defined. Document that we don't do anything in this case.
It turns out that enabling the proper strict environment breaks at least
some old software, so for the moment it's a nop until that can be sorted
out (though that is a very low proprity task).

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