The API declaration in png.h is removed if both READ and WRITE USER_TRANSFORM
are turned off but was left defined in pngtrans.c

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 updated the "last change" comments to 1.5.0