so it will compile when neither PNG_USER_CHUNK_MALLOC_MAX nor
PNG_SET_CHUNK_MALLOC_LIMIT_SUPPORTED is defined.

and removed a redundant line from the copyright info in pngconf.h

from the source files, along with "#define PNG_EXPOSE_INTERNAL_STRUCTURES"
and "#define PNG_NO_PEDANTIC_WARNINGS" (John Bowler).
Also created new pngdebug.h and moved debug definitions there.

and setjmp usage has been improved.

with the help of "cppi".

to look like those in "man man"

This is necessary when libz is upgraded to 1.2.4

and CMakeLists.txt, and changed the name of png_ptr->jmpbuf in pngstruct.h
to png_ptr->png_jmpbuf.

when building on AIX.

otherwise AIX reserves "jmpbuf" and renames it to "__jmpbuf".