- the cpp command is used to process Make/{files,options}, but builtin
  defines such as `linux` will cause problems (macro replacement) if
  these is present in the Make/{files,options}.

  Solve by undefining -Ulinux, -Uunix macros, which will leave directory
  names such as "/usr/lib/x86_64-linux-gnu/..." intact.

  Directories with _linux, __linux__ content (for example), could
  still pose future issues.