add var/macports/build to prefix.mtree so it will always be present and the code that decides whether to build in the home dir won't get confused
add var/macports/build to prefix.mtree so it will always be present and the code that decides whether to build in the home dir won't get confused

git-svn-id: 620571fa9b4bd0cbce9a0cf901e91ef896adbf27@56147 d073be05-634f-4543-b044-5fe20cf6d1d6

neverpanic/macports-base,danchr/macports-base,macports/macports-base,macports/macports-base,neverpanic/macports-base,danchr/macports-base