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

libc/bionic/fork.c

@@ -27,6 +27,7 @@
  */
 #include <unistd.h>
 #include "pthread_internal.h"
+#include "cpuacct.h"
 
 extern int  __fork(void);