>>> "PE" == Paul Eggert <eggert@cs.ucla.edu> writes:

   > On 1/25/20 2:20 PM, Uwe Brauer wrote:
   >> And it is the same problem, although now png16 is installed configure,
   >> sets the flag to png12.

   > What is the output of the following shell commands?

   > pkg-config --cflags libotf
   > pkg-config --libs libotf

   > If they output "-lpng12", then that's most likely the problem since
   > 'configure' uses the output of these commands. You can run
   > './configure --without-libotf' as Stefan suggested, or you can
   > investigate why the pkg-config libotf configuration is messed up for
   > you. I don't observe the problem on Ubuntu 18.04.3 LTS, for what it's
   > worth.

Thanks. The issue was the following, I added in the past some ppa for
non-Ubuntu deb packages, when I then run 

  apt-get upgrade

libpng-dev:amd64 <none> 1.6.34-2~ubuntu16.04.1~ppa1

Was installed, which is not the official Ubuntu version. Although the
library worked  on my distribution, somehow configure got confused,
detected 1.6 but also some remains of 12.

So a simple 

sudo apt-get install libpng12-dev:

Solved the problem, in the sense I, again compile emacs.

Not sure what to think about it. The main culprit is of course Ubuntu,
but that configure got confused is also not good.

Thanks for you help

Uwe