AP_Common: add macro to ignore unused result

In some cases we want to call functions annotated with
warn_unused_result but we know it's safe to ignore the return value.
Prefixing with (void) used to work, but it doesn't do anymore on all
versions of gcc and clang.
See: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66425

This solution is a mix of the solutions provided in the above bug report
and the one provided by Gustavo Sousa at
https://github.com/ArduPilot/ardupilot/pull/4277#issuecomment-224957375

libraries/AP_Common/AP_Common.h

@@ -64,6 +64,27 @@ char (&_ARRAY_SIZE_HELPER(T (&_arr)[0]))[0];
 
 #define ARRAY_SIZE(_arr) sizeof(_ARRAY_SIZE_HELPER(_arr))
 
+/*
+ * See UNUSED_RESULT. The difference is that it receives @uniq_ as the name to
+ * be used for its internal variable.
+ *
+ * @uniq_: a unique name to use for variable name
+ * @expr_: the expression to be evaluated
+ */
+#define _UNUSED_RESULT(uniq_, expr_)                      \
+    do {                                                  \
+        decltype(expr_) uniq_ __attribute__((unused));    \
+        uniq_ = expr_;                                    \
+    } while (0)
+
+/*
+ * Allow to call a function annotated with warn_unused_result attribute
+ * without getting a warning, because sometimes this is what we want to do.
+ *
+ * @expr_: the expression to be evaluated
+ */
+#define UNUSED_RESULT(expr_) _UNUSED_RESULT(__unique_name_##__COUNTER__, expr_)
+
 // @}