Add R__SUGGEST_ALTERNATIVE macro to flag legacy functions

Functions that are outdated, but will not be removed, can be flagged with `R__SUGGEST_ALTERNATIVE`, a macro expanding to ROOT's `_R__DEPRECATED_LATER`. This can be used to point users to the recommended interfaces.

Flagging a function like this:
TIterator* createIterator() const
R__SUGGEST_ALTERNATIVE("begin(), end() and range-based for loops are faster.") {
   ...
}

and defining `R__SUGGEST_NEW_INTERFACE` in a translation unit will trigger a warning such as:
RooChebychev.cxx:66:34: warning: 'createIterator' is deprecated: There is a superior alternative: begin(), end() and range-based for loops are faster. [-Wdeprecated-declarations]
  TIterator* coefIter = coefList.createIterator();

IMPORTANT:
Theoretically, the macro could be used for classes: Don't!
This will trigger warnings for all uses of the class in the interfaces of ROOT.

core/base/inc/ROOT/RConfig.h

@@ -521,6 +521,15 @@
 #define R__DEPRECATED(MAJOR, MINOR, REASON) \
   _R__JOIN3_(_R__DEPRECATED_,MAJOR,MINOR)("will be removed in ROOT v" #MAJOR "." #MINOR ": " REASON)
 
+/* Mechanism to advise users to avoid legacy functions that will not be removed */
+#ifdef R__SUGGEST_NEW_INTERFACE
+#  define R__SUGGEST_ALTERNATIVE(ALTERNATIVE) \
+      _R__DEPRECATED_LATER("There is a superior alternative: " ALTERNATIVE)
+#else
+#  define R__SUGGEST_ALTERNATIVE(ALTERNATIVE)
+#endif
+
+
 /*---- misc ------------------------------------------------------------------*/
 
 #ifdef R__GNU