Bug#751098: The -dev packages need a rework

[Julien Puydt]

Hi,

I just had a look, thinking it was an easy bug to fix, but it turns out 
things are more complicated:

- in fact, it looks like only the polybori-groebner headers use gdlib 
and libpng, so only that .pc should depend on them ;

- the relevant libpolybori-groebner-dev package should depend on 
libdg-dev and libpng-dev (or some such)

I have no time to dive deeper for now.

Snark on #debian-science