updating make targets for models

This patch should fix this sort of error:
```
c++     src/test/mcmc/models/improper.cpp  -Lbin -lstan -o src/test/mcmc/models/improper
src/test/mcmc/models/improper.cpp:3:39: fatal error: stan/model/model_header.hpp: No such file or directory
 #include <stan/model/model_header.hpp>
                                       ^
compilation terminated.
```

I'm not sure what the best way to test this is.