ENH: provide 'sanitized' values for wmake -show compiler information (#1256)

- with the wmake rules we may have some compiler options bound to the
  internal compiler variable. For example,

     CC  = g++ -std=c++11 -m64

     c++FLAGS = ...

  So shift any flags from CC to CXXFLAGS for the output of
  'wmake -show-cxx', 'wmake -show-cxxflags', etc.

  This makes it much easier to handle the values correctly elsewhere.
  Eg,

      CXX="$(wmake -show-cxx)" CXXFLAGS="$(wmake -show-cxxflags)" \
         ./configure