PNG: Fix compilation on some machines
PNG: Fix compilation on some machines

It appears as though environment variable PNG_Z_DEFAULT_COMPRESSION is not defined in all libraries; make use of Z_DEFAULT_COMPRESSION instead.

MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3