HEAD aimake changed; fix the --directory-layout

`--directory-layout=prefix` doesn't work with HEAD anymore. Replace it with `--directory-layout=fhs_package`

This works for me:
```
==> Finishing up
ln -s ../Cellar/nethack4/HEAD/bin/nethack4 nethack4
ln -s ../../Cellar/nethack4/HEAD/share/doc/changelog.txt changelog.txt
ln -s ../../Cellar/nethack4/HEAD/share/doc/faq.txt faq.txt
ln -s ../../Cellar/nethack4/HEAD/share/doc/philosophy.txt philosophy.txt
ln -s ../Cellar/nethack4/HEAD/share/license license
ln -s ../../../Cellar/nethack4/HEAD/share/man/man6/nethack4.6 nethack4.6
ln -s ../Cellar/nethack4/HEAD/share/nhdat nhdat
ln -s ../Cellar/nethack4/HEAD/share/textascii.nh4ct textascii.nh4ct
ln -s ../Cellar/nethack4/HEAD/share/textunicode.nh4ct textunicode.nh4ct
```