\begin{tcolorbox}
  \footnotesize
  \texttt{\mkcomment{Write download URL into the paper (through a LaTeX macro).}}

  \texttt{\mktarget{\$(mtexdir)/download.tex}: \$(\mkvar{indir})/menke20.xlsx}

  \texttt{\mktab{}\mkprog{echo} "\textbackslash{}newcommand{\textbackslash{}menketwentyurl}\{\mktarget{\$(MK20URL)}\}" > \$@}
\end{tcolorbox}