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/isrv.h

@@ -8,6 +8,10 @@
  * Licensed under GPL version 2, see file LICENSE in this tarball for details.
  */
 
+#if __GNUC_PREREQ(4,1)
+# pragma GCC visibility push(hidden)
+#endif
+
 /* opaque structure */
 struct isrv_state_t;
 typedef struct isrv_state_t isrv_state_t;
@@ -31,3 +35,7 @@ void isrv_run(
 	int timeout,
 	int linger_timeout
 );
+
+#if __GNUC_PREREQ(4,1)
+# pragma GCC visibility pop
+#endif