CONFIG: ptscotch header not found on ArchLinux (fixes #1877)

- on ArchLinux, everything is installed under /usr/include/scotch.

  The detection script uses SCOTCH_ARCH_PATH as an initial guess for
  ptscotch as well. However, on the second pass, it has an absolute
  value ("/usr") instead of a logical one ("scotch-system").
  This resulted in the logic for handling scotch+ptscotch subdirs
  being bypassed.