Removing libpng from ext and removed the libpng build command from the ext build target. Added the -DPNG_SKIP_SETJMP_CHECK flag to the linux2-config.py build config file. This should allow users to use their local copy of libpng rather than relying on the libpng in ext. Users should probably clean out their <fife>/etc/install/* directories and re-run 'scons ext'. Tested on ubuntu 10.04