Thanks,
I took a fresh clone of the source from githubs emacs-mirror, and it compiled OK.
It came be closed

mvh
Niels
 <applewebdata://3C97D2C4-8F68-4E6A-BBE6-9F1670A8541B>
Niels Søndergaard
Mariehøj 236, 2990 Nivå
 <tel:+45 4052 2789>+45 4052 2789niels@algon.dk <mailto:niels@algon.dk>
Ultra posse nemo obligatur.

> Den 5. nov. 2021 kl. 21.21 skrev Eli Zaretskii <eliz@gnu.org>:
> 
>> From: Glenn Morris <rgm@gnu.org>
>> Cc: Niels Søndergaard <nisoni@algon.dk>,
>>  51611@debbugs.gnu.org
>> Date: Fri, 05 Nov 2021 15:05:20 -0400
>> 
>> WEBP_CFLAGS is missing from src/Makefile.in.
> 
> Thanks, should be fixed now.
> 
> Niels, please try the latest master branch.