Define SOL_LOCAL if not defined already (e.g. on FreeBSD).

Some evidence that defining it to be 0 is right:
* OS X headers define it to be 0.
* Other code uses 0 instead of SOL_LOCAL to check for peer credentials
  (e.g. FreeBSD's implementation of getpeereid).

src/nix-daemon/nix-daemon.cc

@@ -692,6 +692,10 @@ static PeerInfo getPeerInfo(int remote)
 
 #elif defined(LOCAL_PEERCRED)
 
+#if !defined(SOL_LOCAL)
+#define SOL_LOCAL 0
+#endif
+
     xucred cred;
     socklen_t credLen = sizeof(cred);
     if (getsockopt(remote, SOL_LOCAL, LOCAL_PEERCRED, &cred, &credLen) == -1)