Add atomic compare-and-swap

The atomic compare-and-swap can be used to implement lock-free
structures. The planned use for this is to remove some of the locking done
in the services when listeners are being manipulated.

include/maxscale/atomic.h

@@ -102,4 +102,19 @@ static inline void atomic_synchronize()
 #endif
 }
 
+/**
+ * @brief Atomic compare-and-swap of pointers
+ *
+ * @param variable  Pointer to the variable
+ * @param old_value Pointer to the expected value of @variable
+ * @param new_value Stored value if @c variable is equal to @c old_value
+ *
+ * @return True if @c variable and @c old_value were equal
+ *
+ * @note If GCC __atomic builtins are available, the contents of @c variable are
+ * written to @c old_value if the two are not equal. Do not rely on this behavior
+ * and always do a separate read before attempting a compare-and-swap.
+ */
+bool atomic_cas_ptr(void **variable, void** old_value, void *new_value);
+
 MXS_END_DECLS