CONFIG: ptscotch header not found on RedHat/Fedora (fixes #1904)

- no limit to the number of ways of filing ptscotch libraries.
  RedHat/Fedora/CentOS should look for these directories:

      ptscotch include=/usr/include/openmpi-x86_64
      ptscotch library=/usr/lib64/openmpi/lib

  when MPI_ARCH_PATH=/usr/lib64/openmpi
  and mpicc --showme:compile yields -I/usr/include/openmpi-x86_64