libpng 1.4 and newer versions doesn't define png_infopp_NULL anymore. That causes errors while compiling CIFXImageTools.cpp.
Since that constant was defined like this:
#define png_infopp_NULL (png_infopp)NULL
, I simply substituted all the references to that constant in the aforementioned file with just NULL.
The code compiled successfully on my Linux desktop and I am now a happy user of u3d.