Multiple compilation fixes:

- use C style comments, not C++ style;
- don't use #if with undefined parameter;

src/core/ipv6/ip6.c

@@ -347,7 +347,8 @@ ip6_input(struct pbuf *p, struct netif *inp)
   u8_t nexth;
   u16_t hlen; /* the current header length */
   u8_t i;
-#if IP_ACCEPT_LINK_LAYER_ADDRESSING
+#if 0 /*IP_ACCEPT_LINK_LAYER_ADDRESSING*/
+  @todo
   int check_ip_src=1;
 #endif /* IP_ACCEPT_LINK_LAYER_ADDRESSING */
 

src/include/ipv6/lwip/dhcp6.h

@@ -50,7 +50,7 @@
 
 struct dhcp6
 {
-  //TODO: implement DHCP6
+  /*TODO: implement DHCP6*/
 };
 
 #endif /* LWIP_IPV6_DHCP6 */

src/include/lwip/ip_addr.h

@@ -51,7 +51,7 @@ typedef union {
 
 /** These functions only exist for type-safe conversion from ip_addr_t to
     ip6_addr_t and back */
-#if LWIP_ALLOW_STATIC_FN_IN_HEADER
+#ifdef LWIP_ALLOW_STATIC_FN_IN_HEADER
 static ip6_addr_t* ip_2_ip6(ip_addr_t *ipaddr)
 { return (ip6_addr_t*)ipaddr;}
 static ip_addr_t* ip6_2_ip(ip6_addr_t *ip6addr)