I don't quite follow, but I did try your suggestion of DEFINE => q{"-DMY_FORMAT=\"%.17\""} in the Makefile.PL.
Note that I've altered the .16e to .17, as that's what's needed by the module I'm actually testing.
That does allow the compilation to succeed and I do see this output during the build: 
gcc -c   -s -O2 -DWIN32 -DWIN64 -DCONSERVATIVE -DPERL_TEXTMODE_SCRIPTS
+ -DPERL_IMPLICIT_CONTEXT -DPERL_IMPLICIT_SYS -DUSE_PERLIO -D__USE_MIN
+GW_ANSI_STDIO -fwrapv -fno-strict-aliasing -mms-bitfields -s -O2   -D
+VERSION=\"1.52\" -DXS_VERSION=\"1.52\"  "-IC:\_64\perl530_810\lib\COR
+E"  -DPERL_EXT -DUSE_PPPORT_H "-DMY_FORMAT=\"%.17\"" ListUtil.c

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which, I think, looks right.
However, the XS code that relies on MY_FORMAT being set correctly still fails.
I therefore stuck the following in the XS code to see exactly what MY_FORMAT looks like: 
printf("\n**%s**\n", MY_FORMAT);

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And that prints out **.17**, showing quite clearly that the quotes && the percentage sign have been lost.

This Inline::C script also fails to build: 
use strict;
use warnings;
use Config;

use Inline C => Config =>
  USING => 'ParseRegExp',
  CCFLAGSEX => q["-DMY_FORMAT=\"%.16e\""],
  BUILD_NOISY => 1,
;

use Inline C =><<'EOC';

SV * foo(void) {
  printf(MY_FORMAT, sqrt(2.0));
  printf("\n");
}

EOC

foo();

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During the build I get: 
gcc -c  -iquote"C:/_32/C" -s -O2 -DWIN32 -DWIN64 -DCONSERVATIVE  -DPER
+L_TEXTMODE_SCRIPTS -DPERL_IMPLICIT_CONTEXT -DPERL_IMPLICIT_SYS -DUSE_
+PERLIO -D__USE_MINGW_ANSI_STDIO -fwrapv -fno-strict-aliasing -mms-bit
+fields "-DMY_FORMAT=\"%.16e\"" -s -O2   -DVERSION=\"0.00\" -DXS_VERSI
+ON=\"0.00\"  "-IC:\_64\perl530_810\lib\CORE"   try_pl_d937.c
<command-line>: warning: missing terminating " character
try_pl_d937.xs:1:10: fatal error: EXTERN.h: No such file or directory
 #include "EXTERN.h"
          ^~~~~~~~~~
compilation terminated.

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I am still quite open to testing other suggestions ... though I'm well and truly over trying to work it out for myself.

Cheers,
Rob

In reply to Re^2: Defining an XS symbol in the Makefile.PL (quoting) by syphilis
in thread Defining an XS symbol in the Makefile.PL by syphilis

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