[Linux] Deprecated libpng12

[goddessfreya]

Hello stonesense developers,
In Debian Sid the libpng12 package has been removed and replaced with the newer library libpng16. This causes stonesense to not work on Debian Sid.

Debian Sid has already entered the full freeze and sometime this year it will become the stable repository. When such event happens poor Debian users [who upgrade] wont be capable of using stonesense.

What can be done:
1- Package libpng12.so.0 with stonesense and use LD_LIBRARY_PATH (what i'm currently doing)
2- Compile stonesense with libpng16 (preferable)