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/include/limits.h b/libc/include/limits.h
index c204e4d..1de8ea6 100644
--- a/libc/include/limits.h
+++ b/libc/include/limits.h
@@ -86,7 +86,7 @@
 #include <sys/limits.h>
 
 #if __POSIX_VISIBLE
-#include <arch/syslimits.h>
+#include <sys/syslimits.h>
 #endif
 
 #ifndef PAGESIZE