Work around reported bug where Tk port emits the diagnostic: Error: Target com.apple.configure returned: can't read "sources": no such variable
Work around reported bug where Tk port emits the diagnostic:
Error: Target com.apple.configure returned: can't read "sources": no such variable

Apparently it's possible for darwinports::sources to be unbound in an invocation
of dportsearch.  Not sure why, but this works around the problem and allows
the tk port to build again.

git-svn-id: 620571fa9b4bd0cbce9a0cf901e91ef896adbf27@7815 d073be05-634f-4543-b044-5fe20cf6d1d6

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