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
