CONFIG: improve robustness/flexibility for MPI config handling

- use orte-info to determine prefix/libdir for openmpi.
  This removes a run-time dependency on mpicc, which is actually
  only needed for building with MPI (not running with MPI).
  The corresponding openmpi devel package (deb/rpm) will not
  necessarily be installed on a particular system.

- retain mpicc logic if the new logic using orte-info does not
  deliver an answer. Final fallback to using 'orterun' to
  infer prefix/libdir.

- Additional logic for intel and msmpi to make it easier to
  locate these vendor packages within ThirdParty
  (ie, under ThirdParty/opt/...)

CONFIG: improve robustness

- add check for absolute path when adding PATH/LD_LIBRARY_PATH etc.

- prefix more variables with '_foam*' to prevent accidental overwrite
  of userspace shell variables when sourcing

etc/config.csh/example/prefs.csh

@@ -8,8 +8,7 @@
 #     Copyright (C) 2011-2016 OpenFOAM Foundation
 #------------------------------------------------------------------------------
 # License
-#     This file is part of OpenFOAM, licensed under GNU General Public License
-#     <http://www.gnu.org/licenses/>.
+#     This file is part of OpenFOAM, distributed under GPL-3.0-or-later.
 #
 # File
 #     config.csh/example/prefs.csh