to disable the warnings in pngconf.h

and added "#define PNG_NO_PEDANTIC_WARNINGS" in the libpng source files.

It will be replaced in the future with a function that accounts for gamma.

    longjmp function as well as returning a pointer to the setjmp
    jmp_buf buffer, and marked direct access to jmpbuf 'deprecated'.
    (John Bowler)

    directory, to libpng-config.in and libpng-pc.in, respectively, and
    modified Makefile.am and configure.ac accordingly.  Now "configure"
    needs nothing from the "scripts" directory.
  Also, avoid redefining PNG_CONST in pngconf.h

(ported from the "master" branch)