Delete vestigial declarations formerly used on 16-bit platforms

Delete the structure members used in the 16-bit Turbo C memory models.
Delete the PNG_ABORT declaration used in the 16-bit Windows build.

Stop checking macros (e.g. _WINDOWS) predefined by 16-bit Windows
compilers (but do check the __NT__ macro, which was the only one
predefined by ancient Windows NT compilers).
3 files changed