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/netbsd/net/base64.c b/libc/netbsd/net/base64.c
index 70caaf7..7270703 100644
--- a/libc/netbsd/net/base64.c
+++ b/libc/netbsd/net/base64.c
@@ -141,7 +141,7 @@
 	size_t targsize;
 {
 	size_t datalength = 0;
-	u_char input[3];
+	u_char input[3] = { 0, 0, 0 };  /* make compiler happy */
 	u_char output[4];
 	size_t i;