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

   > On 1/24/20 12:52 AM, Uwe Brauer wrote:
   >> Well it seems that they should say lpng16 and the don't

   > In that case I suggest debugging 'configure' to see why it outputs
   > '-lpng12' even though you haved png16 installed.
I am sure how to debugg configure.

   > In some cases there can be multiple pkg-config installations and
   > perhaps you've picked up the wrong one. You may need to set (or unset)
   > PKG_CONFIG_LIBDIR in your environment, before you run 'configure'.

Config.log looks like this
pkg_cv_CAIRO_CFLAGS='-I/usr/include/cairo -I/usr/include/glib-2.0 -I/usr/lib/x86_64-linux-gnu/glib-2.0/include -I/usr/include/pixman-1 -I/usr/include/freetype2 -I/usr/include/libpng16'
pkg_cv_LIBOTF_LIBS='-lfreetype -lz -lpng12 -lotf -lfreetype'
pkg_cv_PNG_CFLAGS=-I/usr/include/libpng16
pkg_cv_PNG_LIBS='-lpng16 -lz'
CAIRO_CFLAGS='-isystem /usr/include/cairo -isystem /usr/include/glib-2.0 -isystem /usr/lib/x86_64-linux-gnu/glib-2.0/include -isystem /usr/include/pixman-1 -isystem /usr/include/freetype2 -isystem /usr/include/libpng16'
LIBOTF_LIBS='-lfreetype -lz -lpng12 -lotf -lfreetype'


so at some points it gets confused. I insist, the 23rd of Dezember it
was fine, I just checked, indeed 
2019-12-26 11:26:00 install libpng-dev:amd64 <none> 1.6.34-2~ubuntu16.04.1~ppa1
2019-12-26 11:26:01 install libpng-tools:amd64 <none> 1.6.34-2~ubuntu16.04.1~ppa1

the 26th I installed a different png version. I will test now, with I
still can compile master from 23 of December.

Uwe Brauer