libc: tag missing functions in system headers.

This matches recent changes in the NDK header.
We enclose missing functions in #if 0 .. #endif blocks
with a clear "MISSING" in comments in order to locate
them later.

Change-Id: I87b3a62e777897e75c9243360fb0a82bcc53d9fb

libc/include/sched.h

@@ -69,7 +69,7 @@ extern int sched_rr_get_interval(pid_t pid, struct timespec *tp);
 #define CLONE_CHILD_SETTID   0x01000000
 #define CLONE_STOPPED        0x02000000
 
-#ifdef __GNU_SOURCE
+#ifdef _GNU_SOURCE
 extern int clone(int (*fn)(void *), void *child_stack, int flags, void*  arg, ...);
 #endif