#345 Build pdftocairo with libpng support

[Jim]

If you print the usage instructions for pdftocairo, e.g. by running it without arguments, you can see that "-png" is not listed as an option. That is because this ifdef does not evaluate to true; that is, pdftocairo is built without png support. (Also, it is built without tiff support, although this is probably less important.)

This is quite a significant use case of pdftocairo. For example, this led to LyX bug 10718: instant preview of some tikz pictures not working. It appears that libpng is already built as part of MiKTeX so perhaps it is just a matter of flicking a switch. I realise building software is rarely that straightforward though :-) Still, if possible, it would be great if this were enabled.

[Jim]

Confirmed fixed in 6400 (including the downstream problem of preview in LyX). Thanks!