Remove compiler warnings when building Bionic.

Also add missing declarations to misc. functions.
Fix clearerr() implementation (previous was broken).
Handle feature test macros like _POSIX_C_SOURCE properly.

Change-Id: Icdc973a6b9d550a166fc2545f727ea837fe800c4
diff --git a/libc/bionic/cpuacct.c b/libc/bionic/cpuacct.c
index abdbc51..7317073 100644
--- a/libc/bionic/cpuacct.c
+++ b/libc/bionic/cpuacct.c
@@ -29,7 +29,7 @@
 #include <stdio.h>
 #include <errno.h>
 #include <sys/stat.h>
-//#include <sys/types.h>
+#include "cpuacct.h"
 
 int cpuacct_add(uid_t uid)
 {