Sample code (x.c):

int main(void) {
  int return_pcap_debug(void) {
    extern int pcap_debug;

    return pcap_debug;
  }

  return 0;
}

Compiled with (-Os, -O1, -O2 are all the same):

gcc x.c -o x

Produces: 

/tmp/ccJy8Ead.o(.text+0xa): In function `return_pcap_debug.0':
: undefined reference to `pcap_debug'
collect2: ld returned 1 exit status

But, when compiled with:

gcc -O3 x.c -o x

The code compiles.

This is an over-optimisation, and is pretty silly, especially since the returned
value in the inlined block is an extern.

If you link the code against something that does export pcap_debug, it links
properly.