Add a -U option to mkfiles.pl, which is just like -u except that it

runs configure at the top level rather than the unix subdirectory. I'm
getting into the idea of even doing it that way myself, because then I
can do VPATH builds from the same source tree elsewhere.

(Autoconf seems to be fine with doing multiple VPATH builds from the
same source tree in different build directories, but gets upset if you
try to do a VPATH build when you've done a normal build in the real
configure directory. So this way I do what autoconf sees as _only_
VPATH builds.)

git-svn-id: svn://svn.tartarus.org/sgt/putty@9269 cda61777-01e9-0310-a592-d414129be87e