Works fine, fpc 2.0.4, Linux. Just the same old warnings about mixing different types:

Code:
libpng.pas(1630,30) Warning: Mixing signed expressions and longwords gives a 64bit result
libpng.pas(1636,44) Warning: Mixing signed expressions and longwords gives a 64bit result
libpng.pas(1643,30) Warning: Mixing signed expressions and longwords gives a 64bit result
libpng.pas(1649,47) Warning: Mixing signed expressions and longwords gives a 64bit result
I can't see any signed expressions there. Maybe the compiler is too dumb to realise that there are none.