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/include/stdio.h

@@ -305,7 +305,7 @@ char	*cuserid(char *);
 FILE	*fdopen(int, const char *);
 int	 fileno(FILE *);
 
-#if (__POSIX_VISIBLE >= 199209) || 1 /* ANDROID: Bionic does include this */
+#if (__POSIX_VISIBLE >= 199209)
 int	 pclose(FILE *);
 FILE	*popen(const char *, const char *);
 #endif