Grab a constant from the OS?

[Stan Mulder]

I do this successfully for my tagline (the line at the bottom of the last
page.):

lilystring = #(string-append "Engraved in LilyPond " (lilypond-version) " -
Linux")

\header {
  title = "My Song"
  tagline = \markup {
    \column {
      \fontsize #-2 \lilystring
    }
  }
}

It produces this text:

"Engraved in LilyPond 2.19.38 - Linux"

But I would like it to read like this:

"Engraved in LilyPond 2.19.38 - Linux Mint 17.3"

I'm not sure where external constants come from, like grabbing the current
version of the OS. From lilypond or from scheme? I have not worked with
scheme at all, but if it is from scheme, is there a recommended tutorial
that would help with this? And how can I accomplish the immediate goal?

Stan