libpng-1.5 transition

[Adam Golebiowski]

Hi,

there are 3 packages left to finish libpng-1.5 transition:
- nxcompext
- plt
- sng

nxcompext seems to fail due to some libpng unrelated issue.

plt fails to build even with a trivial libpng-1.5 patch applied. However
there's a successor to it - racket.spec. Builds, maybe works, spec needs
some polishing.

Finally there is sng. This one is nasty, it touches so many libpng
privates that it seems impossible to port it to libpng-1.5. I prepared
it to be ready to be built with libpng14.

Anyway, if you care about any of these packages, please act.


-- 
adamg