problems with build scripts that search for config.h and find an
inappropriate one in the standard paths.

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

to prevent warnings about direct access to png structs by libpng functions
while building libpng.  They need to be tested, especially
those using compilers other than gcc.

It is no longer used since the MMX support was removed.