changed semantic of [def] statements:

- [def] is used to define a variable (like before)
  but if we assign a string such as [def var "string"] the string is evaluated to bool
  in a different way: empty string is false, not empty string is true
- added [def?] statement - similar like [def] but define a variable only if such
  a variable is not already defined
- added [let] statement - similar like [def] but with lazy evaluation (it is an alias to a function)
  if it is called to assign a string e.g. [let var "string"] then this has the
  same meaning like [def] - an alias is only created when we assign a function e.g. [let var my_function]
  both [def] and [let] are using the same Vars object
- added [let?] - make an alias but only if such a variable is not already defined
- a std::map of variables moved outside of Generator
  added method void SetVariables(Vars & variables);
- fixed: a result status was not correctly propagated when evaluating [def] statements,
  this was in Call(...) function: last_res from variables was set in Find()
  but later was overwritten by Call(...) called for parameters (recursively)

src/patternparser.h

@@ -5,7 +5,7 @@
  */
 
 /* 
- * Copyright (c) 2007-2018, Tomasz Sowa
+ * Copyright (c) 2007-2021, Tomasz Sowa
  * All rights reserved.
  * 
  * Redistribution and use in source and binary forms, with or without
@@ -175,6 +175,9 @@ private:
 	void ReadDirectiveComment(Item & item);
 	void ReadDirectiveInclude(Item & item);
 	void ReadDirectiveDef(Item & item);
+	void ReadDirectiveDefIfNotSet(Item & item);
+	void ReadDirectiveLet(Item & item);
+	void ReadDirectiveLetIfNotSet(Item & item);
 	void ReadDirectiveFilter(Item & item);
 	void ReadDirectiveEzc(Item & item);
 	void ReadDirectiveOut(Item & item);