SPDX standardizes how source code conveys its copyright and licensing
information. See https://spdx.github.io/spdx-spec/1-rationale/ . SPDX
tags are adopted in many large projects, including things like the Linux
kernel.
Trying to force the XSI version by undefining _GNU_SOURCE can lead
to compilation errors on some systems because of headers expecting
that _GNU_SOURCE is defined.
This commit uses define checks to detect which version we have.
I tried making an overloaded function (int and const char*) instead,
but that led to a warning about one of the variants being unused.
