upstream_sf_base = http://downloads.sourceforge.net/project/libpng
upstream_path = $(OPK_SOURCE)/$(OPK_SOURCE_VERSION_UPSTREAM)
upstream_archive = libpng-$(OPK_SOURCE_VERSION_UPSTREAM).tar.gz
upstream_url = $(upstream_sf_base)/$(upstream_path)/$(upstream_archive)
source_archive = ../$(OPK_SOURCE)-$(OPK_SOURCE_VERSION_UPSTREAM).orig.tar.gz

$(source_archive):
	wget -O '$@' '$(upstream_url)'

source: $(source_archive)