Move ARRAY_LENGTH out of public headers

Exporting unprefixed symbols is a pretty bad idea so don't do that.
Instea of redefining it WL_ARRAY_LENGTH, we just move the define to
our private header.  The scanner generates code that uses ARRAY_LENGTH,
but we can just make it count the number elements and emit an integer
constant instead.

cursor/wayland-cursor.c

@@ -31,6 +31,8 @@
 
 #include "os-compatibility.h"
 
+#define ARRAY_LENGTH(a) (sizeof (a) / sizeof (a)[0])
+
 struct shm_pool {
 	struct wl_shm_pool *pool;
 	int fd;