svg from inkscape — doesn't work in xelatex?

Question

I want to make use of "svg in tex" technology.

Here's a tex:

\documentclass{article}
\usepackage{color}
\usepackage{graphicx}

\newcommand{\executeiffilenewer}[3]{%
    \ifnum\pdfstrcmp{\pdffilemoddate{#1}}%
        {\pdffilemoddate{#2}}>0{\immediate\write18{#3}}
    \fi}

\newcommand{\includesvg}[1]{%
    \executeiffilenewer{#1.svg}{#1.pdf}%
        {inkscape -z -D --file=#1.svg --export-pdf=#1.pdf --export-latex}%
    \input{#1.pdf_tex}}

\begin{document}
\begin{figure}
    \centering
    %\def\svgwidth{\columnwidth}
    \includesvg{svgfig}
    \caption{My SVG Image}
\end{figure}
\end{document}

One can take any svg image.

Then:

pdflatex -shell-escape tex-file

works, but

xelatex -shell-escape tex-file

and

xelatex -shell-escape -8bit tex-file

both give error:

! Undefined control sequence.
\executeiffilenewer #1#2#3->\ifnum \pdfstrcmp 
                                              {\pdffilemoddate {#1}}{\pdffil...
l.19     \includesvg{svgfig}