Warn if developer_dir is set but doesn't exist
Warn if developer_dir is set but doesn't exist

Apparently MacPorts <= 1.8 shipped a config file with developer_dir set to
/Developer, which causes a number of problems now

git-svn-id: 620571fa9b4bd0cbce9a0cf901e91ef896adbf27@90802 d073be05-634f-4543-b044-5fe20cf6d1d6

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