When building with cmake, png_init_io symbol was not there in built libraries, compared with traditional autotools.
While with autotools, those switches can only be used with "make -DPNG_NO_STDIO". But these switches are enabled by default with CMakeLists.txt.
Please consider change it.
CMakeLists.txt was revised in libpng-1.5.6beta03
Please try it and let us know if it meets your expectations.
No response to my 2-year old request for testing the fix. Closing.