Counter-intuitively, defining the PNG_DEBUG macro as 1 (i.e. setting
the debug verbosity level to 1) does *not* cause any of the existing
debug traces to show up. This setting would have worked if we had trace
statements like `png_debug(0, message)`, but we don't have any.
Debug traces do show up for verbosity levels from 2 to 5. The libpng
manual describes in detail how they are meant to be used.
According to the history of the CMake file, as well as the history of
other project files, PNG_DEBUG was never set to a numeric value larger
than 1. In other words, these debug settings inside these build files
never produced any output in any (unmodified) libpng version.
(As for the configure build, PNG_DEBUG has no special treatment there.)
Considering the plethora of alternative methods to pass C preprocessor
options through our build files, scripts and projects onto libpng, we'd
rather discontinue PNG_DEBUG as a dedicated build option.
Removed the include of sys/types.h - apparently unnecessary now on the
platforms on which it happened (all but Mac OS and RISC OS).
Moved the Mac OS test into pngpriv.h (the only place it is used.)
Updated projects/visualc6 and visualc71 with "/d PNG_CONFIGURE_LIBPNG".
They should work but still need to be updated to remove
references to pnggccrd.c or pngvcrd.c and ASM building.
Added README.txt to the beos, cbuilder5, netware, and xcode projects warning
that they need to be updated, to remove references to pnggccrd.c and
pngvcrd.c and to depend on pngpriv.h
