net: libnetlink -- Add #ifdefs to escape system header conflict

Otherwise getting errors like

 | CC       libnetlink.o
 | In file included from libnetlink.c:4:0:
 | ./include/libnetlink.h:34:0: error: "NLMSG_LENGTH" redefined [-Werror]

And make macros a bit more readable.
Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>

include/libnetlink.h

@@ -8,29 +8,49 @@ struct rtattr {
 
 /* Macros to handle rtattributes */
 
-#define RTA_ALIGNTO     4
-#define RTA_ALIGN(len) ( ((len)+RTA_ALIGNTO-1) & ~(RTA_ALIGNTO-1) )
-#define RTA_OK(rta,len) ((len) >= (int)sizeof(struct rtattr) && \
-		(rta)->rta_len >= sizeof(struct rtattr) && \
-		(rta)->rta_len <= (len))
-#define RTA_NEXT(rta,attrlen)   ((attrlen) -= RTA_ALIGN((rta)->rta_len), \
-		(struct rtattr*)(((char*)(rta)) + RTA_ALIGN((rta)->rta_len)))
-#define RTA_LENGTH(len) (RTA_ALIGN(sizeof(struct rtattr)) + (len))
-#define RTA_SPACE(len)  RTA_ALIGN(RTA_LENGTH(len))
-#define RTA_DATA(rta)   ((void*)(((char*)(rta)) + RTA_LENGTH(0)))
-#define RTA_PAYLOAD(rta) ((int)((rta)->rta_len) - RTA_LENGTH(0))
-
-#define NLMSG_ALIGN(len) ( ((len)+NLMSG_ALIGNTO-1) & ~(NLMSG_ALIGNTO-1) )
-#define NLMSG_HDRLEN     ((int) NLMSG_ALIGN(sizeof(struct nlmsghdr)))
-#define NLMSG_LENGTH(len) ((len)+NLMSG_ALIGN(NLMSG_HDRLEN))
-#define NLMSG_SPACE(len) NLMSG_ALIGN(NLMSG_LENGTH(len))
-#define NLMSG_DATA(nlh)  ((void*)(((char*)nlh) + NLMSG_LENGTH(0)))
-#define NLMSG_NEXT(nlh,len)      ((len) -= NLMSG_ALIGN((nlh)->nlmsg_len), \
-		(struct nlmsghdr*)(((char*)(nlh)) + NLMSG_ALIGN((nlh)->nlmsg_len)))
-#define NLMSG_OK(nlh,len) ((len) >= (int)sizeof(struct nlmsghdr) && \
-		(nlh)->nlmsg_len >= sizeof(struct nlmsghdr) && \
-		(nlh)->nlmsg_len <= (len))
-#define NLMSG_PAYLOAD(nlh,len) ((nlh)->nlmsg_len - NLMSG_SPACE((len)))
+#define RTA_ALIGNTO		4
+#define RTA_ALIGN(len)		(((len) + RTA_ALIGNTO - 1) & ~(RTA_ALIGNTO - 1))
+
+#define RTA_OK(rta, len)	((len) >= (int)sizeof(struct rtattr) &&		\
+				 (rta)->rta_len >= sizeof(struct rtattr) &&	\
+				 (rta)->rta_len <= (len))
+
+#define RTA_NEXT(rta, attrlen)	((attrlen) -= RTA_ALIGN((rta)->rta_len),	\
+				 (struct rtattr *)(((char*)(rta)) + RTA_ALIGN((rta)->rta_len)))
+
+#define RTA_LENGTH(len)		(RTA_ALIGN(sizeof(struct rtattr)) + (len))
+#define RTA_SPACE(len)		RTA_ALIGN(RTA_LENGTH(len))
+#define RTA_DATA(rta)		((void *)(((char *)(rta)) + RTA_LENGTH(0)))
+#define RTA_PAYLOAD(rta)	((int)((rta)->rta_len) - RTA_LENGTH(0))
+
+#ifndef NLMSG_ALIGN
+# define NLMSG_ALIGN(len)	(((len) + NLMSG_ALIGNTO - 1) & ~(NLMSG_ALIGNTO - 1))
+#endif
+
+#ifndef NLMSG_HDRLEN
+# define NLMSG_HDRLEN		((int)NLMSG_ALIGN(sizeof(struct nlmsghdr)))
+#endif
+
+#ifndef NLMSG_LENGTH
+# define NLMSG_LENGTH(len)	((len) + NLMSG_ALIGN(NLMSG_HDRLEN))
+#endif
+
+#define NLMSG_SPACE(len)	NLMSG_ALIGN(NLMSG_LENGTH(len))
+
+#ifndef NLMSG_DATA
+# define NLMSG_DATA(nlh)	((void *)(((char *)nlh) + NLMSG_LENGTH(0)))
+#endif
+
+#ifndef NLMSG_NEXT
+# define NLMSG_NEXT(nlh, len)	((len) -= NLMSG_ALIGN((nlh)->nlmsg_len),	\
+				 (struct nlmsghdr *)(((char *)(nlh)) + NLMSG_ALIGN((nlh)->nlmsg_len)))
+#endif
+
+#define NLMSG_OK(nlh, len)	((len) >= (int)sizeof(struct nlmsghdr) &&	\
+				 (nlh)->nlmsg_len >= sizeof(struct nlmsghdr) && \
+				 (nlh)->nlmsg_len <= (len))
+
+#define NLMSG_PAYLOAD(nlh, len)	((nlh)->nlmsg_len - NLMSG_SPACE((len)))
 
 #define NLMSG_NOOP              0x1     /* Nothing.             */
 #define NLMSG_ERROR             0x2     /* Error                */
@@ -39,7 +59,6 @@ struct rtattr {
 
 #define NLMSG_MIN_TYPE          0x10    /* < 0x10: reserved control messages */
 
-
 int parse_rtattr(struct rtattr *tb[], int max, struct rtattr *rta, int len);
 int nlmsg_receive(char *buf, int len, int (*cb)(struct nlmsghdr *));