I just noticed that Makefiles in pkgsrc use a line like:
MASTER_SITES=   ${MASTER_SITE_SOURCEFORGE:=ffmpeg/}
However the variable modifier := is not described in the make command documentation.
It looks like a subform of :old_string=new_string substitution, where I am guessing the
new_string is just tacked on. Is this correct? If so, this should be added to the documentation,
as it is not obvious that this is what would happen with a blank old_string.