homebrew: avoid pruning out the statedir
homebrew: avoid pruning out the statedir

Avoid this problem:

```
$ brew install -v --HEAD watchman
...
==> make install
/usr/bin/install -c -d -m 777 /usr/local/Cellar/watchman/HEAD/var/run/watchman
 ./install-sh -c -d '//usr/local/Cellar/watchman/HEAD/bin'
 ./install-sh -c -d '//usr/local/Cellar/watchman/HEAD/share/doc/watchman-3.0.0'
 /usr/bin/install -c -m 644 README.markdown '//usr/local/Cellar/watchman/HEAD/share/doc/watchman-3.0.0'
  /usr/bin/install -c watchman '//usr/local/Cellar/watchman/HEAD/bin'
==> Cleaning
rmdir: /usr/local/Cellar/watchman/HEAD/var/run/watchman (empty)
rmdir: /usr/local/Cellar/watchman/HEAD/var/run (empty)
rmdir: /usr/local/Cellar/watchman/HEAD/var (empty)
```

https://github.com/facebook/react-native/issues/239#issuecomment-86713656

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