yacc dependency missing

```
$ make
/bin/bash ./etc/ylwrap parse.y y.tab.c parse.c y.tab.h `echo parse.c | sed -e s/cc$/hh/ -e s/cpp$/hpp/ -e s/cxx$/hxx/ -e s/c++$/h++/ -e s/c$/h/` y.output parse.output -- yacc  -d
./etc/ylwrap: line 176: yacc: command not found
Makefile:675: recipe for target 'parse.c' failed
make: *** [parse.c] Error 127
```

I reckon this should be handled at the `autoconf` stage.