Add helper macro which enables function-like macro overload with 1 or…

… 2 arguments.

Inspired by https://stackoverflow.com/a/11763277/

Warning: This requires support for variadic macros (`__VA_ARGS__`). These were officially added in C99 and C++11.

include/etl/macros.h

@@ -43,5 +43,31 @@ SOFTWARE.
 #define ETL_U16_STRING(X)  ETL_CONCAT(u, ETL_STRINGIFY(X))
 #define ETL_U32_STRING(X)  ETL_CONCAT(U, ETL_STRINGIFY(X))
 
+/**
+ * Resolves a function-like macro overload with 1 or 2 arguments.
+ *
+ * This macro can be used to call other function-like macros depending on the number of arguments
+ * provided by the caller.
+ *
+ * \param _1 is the first argument given to the called macro
+ * \param _2 is either the second argument given to the called macro
+ *           or the name of the macro which takes to arguments
+ * \param SELECTED_MACRO_NAME is the name of the macro to be called
+ * \param ... is a non-empty list of arguments:
+ *        The beginning of the list is a list of all macro names which accept less arguments than
+ *        `SELECTED_MACRO_NAME`. This list may be empty.
+ *        The last element in the list is a dummy argument. For example `""`.
+ *        For compatibility reasons `...` should not be empty.
+ *
+ * Use this function-like macro as follows:
+ *
+ *     #define TAKES1( a ) (a)    //!< example macro which takes 1 argument
+ *     #define TAKES2(a, b) (a+b) //!< example macro which takes 2 arguments
+ *     #define TAKES_1or2(...) GET_MACRO_OVERLOAD2(__VA_ARGS__, TAKES2, TAKES1, "")(__VA_ARGS__)
+ *
+ * Now `TAKES_1or2()` can be called with 1 or 2 arguments.
+ */
+#define GET_MACRO_OVERLOAD2(_1, _2, SELECTED_MACRO_NAME, ...) SELECTED_MACRO_NAME
+
 #endif