Use intra-DSO calls when we build libbusybox. No-op for normal build.

This was Bernhard Fischer' idea.

   text    data     bss     dec     hex filename
 773549    7781    9740  791070   c121e busybox.t7/0_lib/libbusybox.so.1.11.0.svn_unstripped
 769683    7397    9740  786820   c0184 busybox.t9t/0_lib/libbusybox.so.1.11.0.svn_unstripped

networking/libiproute/libnetlink.h

@@ -8,6 +8,11 @@
 #include <linux/netlink.h>
 #include <linux/rtnetlink.h>
 
+
+#if __GNUC_PREREQ(4,1)
+# pragma GCC visibility push(hidden)
+#endif
+
 struct rtnl_handle
 {
 	int			fd;
@@ -43,4 +48,8 @@ extern int rta_addattr_l(struct rtattr *rta, int maxlen, int type, void *data, i
 
 extern int parse_rtattr(struct rtattr *tb[], int max, struct rtattr *rta, int len);
 
+#if __GNUC_PREREQ(4,1)
+# pragma GCC visibility pop
+#endif
+
 #endif /* __LIBNETLINK_H__ */