Front End (part 2)

More easy stuff.
Makefiles have a pretty simple syntactic structure.
Our parser is recursive-descent, extended LL(1).
It is written in the  pa_extend extension language of camlp4 
This language is designed for extending the Ocaml grammar.
But we are just going to replace the grammar altogether.

EXTEND
makefile_item:
         [ [ EOL -> None
           | s = WORD; "="; ws = words; EOL -> ... 
           | t = WORD; ":"; ws = words; EOL; a = actions -> ... ] ]; 
actions: [ [ az = LIST0 action -> ... ]  ];
action:  [ [ TAB; ws = words; EOL -> ... ] ];
words:   [ [ ws = LIST0 word -> ... ] ];
word:    [ [ w = WORD -> ...
           | "$"; "^" -> ...
           | "$"; "<" -> ...
           | "$"; "@" -> ...
           | "$"; "("; w = WORD; ")" -> ... ] ];
END