A user points out that Loopy's solver relies on elements of the

`clues' array being able to be -1, so we must explicitly declare it
as `signed char' or it will break on platforms whose default char is
unsigned.

git-svn-id: svn://svn.tartarus.org/sgt/puzzles@7636 cda61777-01e9-0310-a592-d414129be87e