Completely revamped mkfiles.pl which incorporates dependency

Completely revamped mkfiles.pl which incorporates dependency
analysis (for both .c and .rc files). Generates the VC++ makefile as
well as the other two; the authoritative source is now the new file
`Recipe' rather than any particular Makefile. Note that `Makefile'
is still here as a relic of the old way until we stop the nightly
builds using it, but it'll be gone soon.

git-svn-id: http://svn.tartarus.org/sgt/putty@1592 cda61777-01e9-0310-a592-d414129be87e