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

include/rtc_.h

@@ -9,12 +9,14 @@
 
 #include "libbb.h"
 
+#if __GNUC_PREREQ(4,1)
+# pragma GCC visibility push(hidden)
+#endif
+
 extern int rtc_adjtime_is_utc(void);
 extern int rtc_xopen(const char **default_rtc, int flags);
 extern time_t rtc_read_time(int fd, int utc);
 
-
-
 /*
  * Everything below this point has been copied from linux/rtc.h
  * to eliminate the kernel header dependency
@@ -70,4 +72,8 @@ struct linux_rtc_wkalrm {
 #define RTC_AF 0x20
 #define RTC_UF 0x10
 
+#if __GNUC_PREREQ(4,1)
+# pragma GCC visibility pop
+#endif
+
 #endif