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/udhcp/common.h

@@ -11,6 +11,12 @@
 #define _COMMON_H
 
 #include "libbb.h"
+#include <netinet/udp.h>
+#include <netinet/ip.h>
+
+#if __GNUC_PREREQ(4,1)
+# pragma GCC visibility push(hidden)
+#endif
 
 #define DEFAULT_SCRIPT   CONFIG_DHCPC_DEFAULT_SCRIPT
 
@@ -18,9 +24,6 @@ extern const uint8_t MAC_BCAST_ADDR[6]; /* six all-ones */
 
 /*** packet.h ***/
 
-#include <netinet/udp.h>
-#include <netinet/ip.h>
-
 #define DHCP_OPTIONS_BUFSIZE  308
 
 struct dhcpMessage {
@@ -98,4 +101,8 @@ int arpping(uint32_t test_ip, uint32_t from_ip, uint8_t *from_mac, const char *i
 # define DEBUG(str, args...) do {;} while (0)
 #endif
 
+#if __GNUC_PREREQ(4,1)
+# pragma GCC visibility pop
+#endif
+
 #endif