add -fvisibility=hidden to CC flags, mark XXX_main functions

EXTERNALLY_VISIBLE. 5% size reduction of libbusybox.so

procps/watch.c

@@ -23,7 +23,7 @@
 //
 // (procps 3.x and procps 2.x are forks, not newer/older versions of the same)
 
-int watch_main(int argc, char **argv);
+int watch_main(int argc, char **argv) MAIN_EXTERNALLY_VISIBLE;
 int watch_main(int argc, char **argv)
 {
 	unsigned opt;