Will attach patch, see upstream bug report.
Created attachment 263275 [details, diff] inkscape-0.48.1-libpng15.diff Makes inkscape build against libpng 1.5. Works against both 0.48.0 and 0.48.1.
Fixed in 0.48.1 in CVS. I didn't add it to inkscape-0.48.0 since 0.48.1 will probably go stable before libpng-1.5. :)