I created this ticket with the solution.

Tested on Windows 10 with Strawberry 5.32

--- encGlue.c.org       2020-09-01 17:34:14.514068300 +0200
+++ encGlue.c   2020-09-01 17:42:48.710327700 +0200
@@ -39,6 +39,9 @@
 #define NEED_PUSHSTACK_HACK
 #endif

+#undef  MAX
+#define MAX(a,b) ((a)>(b)?(a):(b))
+
 /* ------------------------------------------------------------------
+-------- */
 /* UTF8-ness routines
 /* ------------------------------------------------------------------
+-------- */
[download]

So, start cmd and:

C:> cd \Strawberry\cpan\build\Tk-804.035.0
C:\Strawberry\cpan\build\Tk-804.035.0> vim encGlue.c
-- make the above change
C:\Strawberry\cpan\build\Tk-804.035.0> gmake
C:\Strawberry\cpan\build\Tk-804.035.0> gmake test
C:\Strawberry\cpan\build\Tk-804.035.0> gmake install
[download]

Update 2020-09-08: This fix is queued for the next release of Tk.

I can confirm that that code edit was sufficient for to get Tk-804.035 to install OK for me.

Many thanks!

Thank you very much. The problem with MAX is solved now. But unfortunately I have more undefined references.

Here the output of my log:

C:/Strawberry/c/bin/../lib/gcc/x86_64-w64-mingw32/8.3.0/../../../../x8
+6_64-w64-mingw32/bin/ld.exe: encGlue.o:encGlue.c:(.text+0x45f): undef
+ined reference to `Perl_to_utf8_lower'
C:/Strawberry/c/bin/../lib/gcc/x86_64-w64-mingw32/8.3.0/../../../../x8
+6_64-w64-mingw32/bin/ld.exe: encGlue.o:encGlue.c:(.text+0x4bf): undef
+ined reference to `Perl_to_utf8_upper'
C:/Strawberry/c/bin/../lib/gcc/x86_64-w64-mingw32/8.3.0/../../../../x8
+6_64-w64-mingw32/bin/ld.exe: encGlue.o:encGlue.c:(.text+0x98): undefi
+ned reference to `Perl_is_uni_alpha'
C:/Strawberry/c/bin/../lib/gcc/x86_64-w64-mingw32/8.3.0/../../../../x8
+6_64-w64-mingw32/bin/ld.exe: encGlue.o:encGlue.c:(.text+0xb8): undefi
+ned reference to `Perl_is_uni_alnum'
C:/Strawberry/c/bin/../lib/gcc/x86_64-w64-mingw32/8.3.0/../../../../x8
+6_64-w64-mingw32/bin/ld.exe: encGlue.o:encGlue.c:(.text+0xd8): undefi
+ned reference to `Perl_is_uni_space'
C:/Strawberry/c/bin/../lib/gcc/x86_64-w64-mingw32/8.3.0/../../../../x8
+6_64-w64-mingw32/bin/ld.exe: encGlue.o:encGlue.c:(.text+0xf8): undefi
+ned reference to `Perl_is_uni_upper'
collect2.exe: error: ld returned 1 exit status
gmake: *** [Makefile:617: blib\arch\auto\Tk\Tk.xs.dll] Error 1
  SREZIC/Tk-804.034.tar.gz
  C:\STRAWB~1\c\bin\gmake.exe -- NOT OK
Stopping: 'install' failed for 'Tk'.
Failed during this command:
 SREZIC/Tk-804.034.tar.gz                     : make NO
[download]

Do you have any idea what could be the problem here?

Your notes show you are installing an old version, Tk-804.034. See rt://125665. When installing check the open tickets, closed tickets impacting the version you are trying to work wth, changes etc. Metacpan makes this straightforward. Tk.

I see. I have to use Tk-804.035 instead of Tk-804.034 to get rid of these errors.

Thank you for your message that's help me on Strawberry 5.32.0.1. Best Regards !

Thank you for the patch!

Confirmed that this fix still works as of 15 Jan 2021

https://cpan.metacpan.org/authors/id/S/SR/SREZIC/Tk-804.035.tar.gz

Sorry for the ignorance. Much thanks, but now how do I install it? I am used to using PPM to do package installs.