I found solution, but I do not know reason yet.
configure calls raydium/php/configure with --with-gd
Removing it and configure will pass.
But there might be good reason to have --with-gd, so I found another solution.
sudo ln -s /usr/lib/x86_64-linux-gnu/libpng.so /usr/lib/libpng.so
(Where also libjpeg caused problems.)
I do not know libgd, libpng (or it's ubuntu packaging) or php enough to say who's fault this is (=where to report).
"--with-png-dir" adds automatically suffix /lib. This can changed with "--with-libdir= ". e.g. "--with-libdir=lib64" or "--with-libdir=i386-linux-gnu", generally speaking:
--with-libdir=`pkg-config --libs libpng | tail -c +12 | head -c -11`
But this is not working on stock Natty (.e.g autobuilder), because then it doesn't find libjpeg, which is installed on /usr/lib.