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

EXTERNALLY_VISIBLE. 5% size reduction of libbusybox.so

networking/wget.c

@@ -391,7 +391,7 @@ static char *gethdr(char *buf, size_t bufsiz, FILE *fp /*, int *istrunc*/)
 }
 
 
-int wget_main(int argc, char **argv);
+int wget_main(int argc, char **argv) MAIN_EXTERNALLY_VISIBLE;
 int wget_main(int argc, char **argv)
 {
 	char buf[512];