fixup build errors/warnings

http://bugs.gentoo.org/240750

--- Makefile
+++ Makefile
@@ -1,4 +1,5 @@
 PREFIX ?= /usr/local
+CPPFLAGS += -D_GNU_SOURCE
 
 all: ssmping asmping ssmpingd mcfirst
 
--- ssmping.c
+++ ssmping.c
@@ -22,7 +22,7 @@
     uint16_t size;
     uint32_t intface;
     struct sockaddr_storage name, ucaddr, mcaddr, grpaddr;
-    size_t namelen;
+    socklen_t namelen;
 #ifdef WIN32
     WORD wVersionRequested;
     WSADATA wsaData;