Testing for variant/version

[Peter Flynn]

A user has suggested the following code to make sure that a  macro is 
using the "right" string comparator macro:

\ifdefined\pdftexversion
   \let\switch at strcmp=\pdfstrcmp
\else
   \ifdefined\xetexversion
     \let\switch at strcmp=\strcmp
   \else
     \ifdefined\luatexversion
       \input pdftexcmds.sty
       \let\switch at strcmp=\pdf at strcmp
     \else
       \let\switch at strcmp=\pdfstrcmp% Danger, Will Robinson!
       \PackageWarningNoLine{stringswitch}{\protect\pdfstrcmp\space 
selected (no \protect\TeX\space variant version found, maybe very old?)}%
     \fi
   \fi
\fi

The final \else was added because TeX Live 2019/Debian in its xelatex 
incarnation was failing the first three tests. Is there another test 
than can be performed for earlier versions (some of them are probably 
still widespread)?

Peter