... % is also special in make's language — it is used in forming patterns ...

Yes, I know that, and it may well be part of the problem.
However, when I run the script on my Ubuntu box (where GNU make is also used) I find that it needs only the single "%", same as dmake.
Can we therefore assume that the bug is in the Win32 version of "make" that I'm using ?

On windows, I've been using: 
GNU Make 3.82.90
Built for i686-pc-mingw32
Copyright (C) 1988-2012 Free Software Foundation, Inc.

[download]
and it makes no difference when I switch to: 
GNU Make 4.2.1
Built for x86_64-pc-msys
Copyright (C) 1988-2016 Free Software Foundation, Inc.

[download]
So, if the problem lies with GNU make on Windows, it seems it's a thoroughly embedded problem.

I know that "%%" is the way to escape the "%" in (s)printf's formatting pattern.
According to https://www.cmcrossroads.com/article/gnu-make-escaping-walk-wild-side, in GNU make one escapes the "%" with a backslash - which was another approach I had tried, and found to be unsuccessful.
I'll investigate the possibility that somewhere in EU::MM there's a (s)printf call that's made when make=gmake, but not when make=dmake. (However, I think it unlikely that would happen.)

The problem of how to workaround the issue is, in my view, solved.
I'd just like to understand why and how that issue exists.

Cheers,
Rob

In reply to Re^3: Defining an XS symbol in the Makefile.PL (largely solved) by syphilis
in thread Defining an XS symbol in the Makefile.PL by syphilis

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