[hjl@gnu-6 needed-5]$ cat foo.c
extern void bar ();

void
foo ()
{
  bar ();
}
[hjl@gnu-6 needed-5]$ cat bar.c
void
bar ()
{
}
[hjl@gnu-6 needed-5]$ cat main.c
extern void foo ();

int
main ()
{
  foo ();
  return 0;
}
[hjl@gnu-6 needed-5]$ make foo
gcc  -O -g   -c -o main.o main.c
gcc  -O -g -fPIC   -c -o bar.o bar.c
gcc  -shared -o libbar.so bar.o
gcc  -O -g -fPIC   -c -o foo.o foo.c
gcc  -shared -o libfoo.so foo.o
gcc  -o foo main.o -Wl,--as-needed libfoo.so libbar.so -Wl,--no-as-needed
-Wl,-rpath,.
libfoo.so: undefined reference to `bar'
collect2: ld returned 1 exit status
make: *** [foo] Error 1
[hjl@gnu-6 needed-5]$