Move subset of libpng and zlib to src
Move subset of libpng and zlib to src; eliminate HAVE_LIBPNG ifdefs

Also update linux/build scripts to exclude local libpng and zlib source files
from compilation. The zlib files won't compile with g++.
Move subset of libpng and zlib to src; eliminate HAVE_LIBPNG ifdefs

Update Visual Studio files accordingly.