Add a wrapper script called 'configure' at the top level, which runs

Add a wrapper script called 'configure' at the top level, which runs
the real configure script from the unix subdirectory, but with cwd
unchanged so that you end up doing a VPATH build in the top-level
source directory.

Should, ideally, placate the people who expect 'configure' to be at
the top level, while still letting _me_ keep all the Unix-specific
stuff in the Unix subdirectory.

git-svn-id: http://svn.tartarus.org/sgt/putty@9241 cda61777-01e9-0310-a592-d414129be87e