Unified multiple definitions of container_of() macro.

Removed duplicate definitions of the container_of() macro and
refactored sources to use the single implementation.

Signed-off-by: Jon A. Cruz <jonc@osg.samsung.com>
Reviewed-by: Peter Hutterer <peter.hutterer@who-t.net>
Reviewed-by: Bryce Harrington <bryce@osg.samsung.com>

shared/config-parser.c

@@ -39,10 +39,7 @@
 
 #include <wayland-util.h>
 #include "config-parser.h"
-
-#define container_of(ptr, type, member) ({				\
-	const __typeof__( ((type *)0)->member ) *__mptr = (ptr);	\
-	(type *)( (char *)__mptr - offsetof(type,member) );})
+#include "helpers.h"
 
 struct weston_config_entry {
 	char *key;

shared/helpers.h

@@ -52,6 +52,43 @@ extern "C" {
 #define MIN(x,y) (((x) < (y)) ? (x) : (y))
 #endif
 
+/**
+ * Returns a pointer the the containing struct of a given member item.
+ *
+ * To demonstrate, the following example retrieves a pointer to
+ * `example_container` given only its `destroy_listener` member:
+ *
+ * @code
+ * struct example_container {
+ *     struct wl_listener destroy_listener;
+ *     // other members...
+ * };
+ *
+ * void example_container_destroy(struct wl_listener *listener, void *data)
+ * {
+ *     struct example_container *ctr;
+ *
+ *     ctr = wl_container_of(listener, ctr, destroy_listener);
+ *     // destroy ctr...
+ * }
+ * @endcode
+ *
+ * @param ptr A valid pointer to the contained item.
+ *
+ * @param type A pointer to the type of content that the list item
+ * stores. Type does not need be a valid pointer; a null or
+ * an uninitialised pointer will suffice.
+ *
+ * @param member The named location of ptr within the sample type.
+ *
+ * @return The container for the specified pointer.
+ */
+#ifndef container_of
+#define container_of(ptr, type, member) ({				\
+	const __typeof__( ((type *)0)->member ) *__mptr = (ptr);	\
+	(type *)( (char *)__mptr - offsetof(type,member) );})
+#endif
+
 #ifdef  __cplusplus
 }
 #endif