* include/hberrors.h
added WARN_VAR_NOT_USED
* source/compiler/harbour.y
added warning message "Variable \'%s\' declared but not used in function: %s"
added linked list FunVars to record the usage of declared variables
added logic to maintain FunVars
added logic to check usage as per FunVars in GenReturn()
* include/hberrors.h
added define WARN_AMBIGUOUS_VAR 1
added declaration for new function GenWarning()
* source/compiler/harbour.y
added _szWarnings[] to include parse warning messages
added function GenWarning()
added /w compiler switch to enable warnings
added warning message for ambiguous vars
