Fine, return symbol if defined, but what if it's empty ? make sure
to return something then!

Index: RawIP.xs
--- RawIP.xs.orig
+++ RawIP.xs
@@ -275,7 +275,7 @@ constant(name, arg)
         case 'l':
         if (strEQ(name, "lib_pcap_h"))
 #ifdef lib_pcap_h
-            return lib_pcap_h;
+            return lib_pcap_h+0;
 #else
         goto not_there;
 #endif