it so BCPPCompiler actually works, so I'm provisionally accepting it
-- ugly and working is better than not working!  Major changes:
  - normalize paths (apparently BC++ doesn't like slashes)
  - overhauled how we search for and specify libraries on the linker
    command-line
  - hacked up 'find_library_file()' so it knows about "debug" library
    naming convention as well as "bcpp_xxx.lib" -- the question is,
    is this a well-established and sensible convention?
Also:
  - change to use 'util.write_file()' to write the .def file