Add and use constants for the Android API levels.

Test: bionic tests still pass
Change-Id: If1b619cfc9db1d3f5d91f14f2ace71058dca0c4a
diff --git a/libc/include/signal.h b/libc/include/signal.h
index 17bc57d..7ae50de 100644
--- a/libc/include/signal.h
+++ b/libc/include/signal.h
@@ -118,7 +118,7 @@
 
 int siginterrupt(int, int);
 
-#if __ANDROID_API__ >= 21
+#if __ANDROID_API__ >= __ANDROID_API_L__
 sighandler_t signal(int, sighandler_t) __INTRODUCED_IN(21);
 int sigaddset(sigset_t*, int) __INTRODUCED_IN(21);
 int sigdelset(sigset_t*, int) __INTRODUCED_IN(21);