snc: deprecated foreign declarations

Foreign declarations have been a bad idea. This became clear after
implementing indirect calls, because as a consequence you get 'used but
not declared' warnings for calls to foreign functions, too. I could
'fix' that with a bit of effort, but that would be misplaced. The C
compiler will warn about undeclared variables and functions anyway, so
why bother? If you really want to know, perhaps for debugging, use -W
option for extra warnings. If you do, you'll now get the more precise
waring 'treating undeclared object xxx as foreign'.