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/fts.c b/libc/bionic/fts.c
index 3dcfb28..c7770b6 100644
--- a/libc/bionic/fts.c
+++ b/libc/bionic/fts.c
@@ -548,9 +548,9 @@
 	DIR *dirp;
 	void *oldaddr;
 	size_t len, maxlen;
-	int nitems, cderrno, descend, level, nlinks, nostat, doadjust;
+	int nitems, cderrno, descend, level, nlinks, nostat = 0, doadjust;
 	int saved_errno;
-	char *cp;
+	char *cp = NULL;
 
 	/* Set current node pointer. */
 	cur = sp->fts_cur;