Conditionally expose splice(2) and sync_file_range(2) constants.

As in other places, only expose the constants at API levels where
we're also exposing the functions (to cope with "poor man's configure"
where an author assumes the presence of the #define to imply the
availability of the function).

Bug: http://b/63145226
Test: builds
Change-Id: I524f7288513aa6c35479c550e748a619397929b0
diff --git a/libc/include/fcntl.h b/libc/include/fcntl.h
index 652430c..c98022c 100644
--- a/libc/include/fcntl.h
+++ b/libc/include/fcntl.h
@@ -56,14 +56,18 @@
 #define O_ASYNC FASYNC
 #define O_RSYNC O_SYNC
 
+#if __ANDROID_API__ >= __ANDROID_API_L__
 #define SPLICE_F_MOVE 1
 #define SPLICE_F_NONBLOCK 2
 #define SPLICE_F_MORE 4
 #define SPLICE_F_GIFT 8
+#endif
 
+#if __ANDROID_API__ >= __ANDROID_API_O__
 #define SYNC_FILE_RANGE_WAIT_BEFORE 1
 #define SYNC_FILE_RANGE_WRITE 2
 #define SYNC_FILE_RANGE_WAIT_AFTER 4
+#endif
 
 int creat(const char*, mode_t);
 int creat64(const char*, mode_t) __INTRODUCED_IN(21);