[hjl@gnu-cfl-1 lto-6]$ cat foo.c 
extern void bar (void) __attribute__((weak));

void
foo (void)
{
  if (bar)
    bar ();
}
[hjl@gnu-cfl-1 lto-6]$ cat lib.c
int FLAGS_verbose;
[hjl@gnu-cfl-1 lto-6]$ cat x.c 
extern int FLAGS_verbose;
extern void bar (void);

int
a(const char *b) {
return FLAGS_verbose;
}
void unused (void) { bar(); }
int main() { a(""); return 0; }
[hjl@gnu-cfl-1 lto-6]$ make clean
rm -f *.o *.so x *.out ./*.s ./*.res
[hjl@gnu-cfl-1 lto-6]$ cat foo.c 
extern void bar (void) __attribute__((weak));

void
foo (void)
{
  if (bar)
    bar ();
}
[hjl@gnu-cfl-1 lto-6]$ cat lib.c 
int FLAGS_verbose;
[hjl@gnu-cfl-1 lto-6]$ cat x.c
extern int FLAGS_verbose;
extern void bar (void);
int
a(void) {
return FLAGS_verbose;
}
void unused (void) { bar(); }
int main() { return a (); }
[hjl@gnu-cfl-1 lto-6]$ make
gcc -B./ -O -flto   -c -o x.o x.c
gcc -B./ -O   -c -o lib.o lib.c
gcc -B./ -O -fpic   -c -o foo.o foo.c
gcc -B./ -shared -O -o libfoo.so foo.o
gcc -B./ -shared -O -o libx.so lib.o libfoo.so
gcc -B./ -o x x.o libx.so -Wl,--as-needed -L. -lfoo
./ld: ./libfoo.so: undefined reference to `bar'
collect2: error: ld returned 1 exit status
make: *** [Makefile:20: x] Error 1
[hjl@gnu-cfl-1 lto-6]$ readelf -s ./libfoo.so | grep bar
     2: 0000000000000000     0 NOTYPE  WEAK   DEFAULT  UND bar

Weak undefined reference is OK.