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

diff --git a/include/unarchive.h b/include/unarchive.h
index b49e6cb..a6b0477 100644
--- a/include/unarchive.h
+++ b/include/unarchive.h
@@ -2,6 +2,10 @@
 #ifndef	__UNARCHIVE_H__
 #define	__UNARCHIVE_H__
 
+#if __GNUC_PREREQ(4,1)
+# pragma GCC visibility push(hidden)
+#endif
+
 #define ARCHIVE_PRESERVE_DATE           1
 #define ARCHIVE_CREATE_LEADING_DIRS     2
 #define ARCHIVE_EXTRACT_UNCONDITIONAL   4
@@ -126,4 +130,8 @@
 #define open_transformer(src_fd, transformer, transform_prog) open_transformer(src_fd, transform_prog)
 #endif
 
+#if __GNUC_PREREQ(4,1)
+# pragma GCC visibility pop
+#endif
+
 #endif