[Gtkradiant] libpng15 support

[Rudolf Polzer]

On Sun, Mar 11, 2012 at 07:25:27PM -0400, Andre D wrote:
> Arch Linux has libpng15 as the current libpng and gtkradiant svn would
> no longer build.  Attached is a patch which should correct the issue.

FYI, this only affects ZeroRadiant. NetRadiant already has that fix and it has
been tested successfully.

This also means it should be safe to apply for ZeroRadiant.

Best regards,

Rudolf Polzer