copypng is missing

[lilyball]

The CopyPNGFile rule invokes a tool copypng to do the actual copying. Xcode's version of copypng here is a Perl script that optionally invokes pngcrush. I expected xcbuild to have something similar, but xcbuild doesn't seem to bundle a copypng at all, so the CopyPNGFile rule simply fails.

[lilyball]

I'm not sure offhand what xcbuild should do about pngcrush here. Perhaps just assume it's in the path if compression is requested. Apple's copypng also adds the -iphone flag but I think that's specific to Apple's bundled pngcrush so a copypng provided by xcbuild should probably skip that flag.