pngdefs.h doesn't seem to exist in any package in the unstable branch of the Debian archive. Actually, pngconf.h does not longer reference it. stellarium 0.8.2-3 is already in Sid, and it correctly builds.
I think this bug should be closed. Thank you very much for your time and good work, Jose -- José Miguel Parrella Romero -> Debian Sid, k2.6.18 Escuela de Computación - Facultad de Ciencias Universidad Central de Venezuela -> ucvlug.info