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/unistd/seteuid.c b/libc/unistd/seteuid.c
index dd94932..b3ea372 100644
--- a/libc/unistd/seteuid.c
+++ b/libc/unistd/seteuid.c
@@ -26,6 +26,9 @@
  * SUCH DAMAGE.
  */
 #include <unistd.h>
+#include "cpuacct.h"
+
+extern int __setresuid(uid_t, uid_t, uid_t);
 
 int seteuid(uid_t euid)
 {