ENH: add wmake -debug option

- forces c++DBUG='-DFULLDEBUG -g -O0' for the compilation, to allow
  localized debugging during development without file editing and
  while retaining the WM_COMPILE_OPTION (eg, Opt)

  Note that switching between 'wmake' and 'wmake -debug' will not
  cause existing targets to be rebuilt. As before, these are driven by
  the dependencies. An intermediate wclean may thus be required.