Reconfigure required for head.

Hi all, I have just committed 4de585a which lifts the MAX_PATH restrictions for Haskell programs compiled with GHC 8.6 for Windows users. However when you update, you *MUST* run configure again. This counts for ALL platforms otherwise your build will suffer a not so friendly failure. If you're using Hadrian to build, you must manually update the submodule to HEAD of master as the Hadrian submodule hasn't been bumped yet. Thanks, Tamar