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.sh/settings

@@ -143,14 +143,14 @@ export FOAM_USER_LIBBIN="$WM_PROJECT_USER_DIR/platforms/$WM_OPTIONS/lib"
 
 
 # Prepend wmake to the path - not required for runtime-only environment
-foundDir="$WM_PROJECT_DIR/wmake"
+_foamFoundDir="$WM_PROJECT_DIR/wmake"
 if [ -d "$WM_DIR" ]
 then
-    foundDir="${WM_DIR}"
+    _foamFoundDir="${WM_DIR}"
 fi
-if [ -d "$foundDir" ]
+if [ -d "$_foamFoundDir" ]
 then
-    PATH="$foundDir:$PATH"
+    PATH="$_foamFoundDir:$PATH"
 else
     unset WM_DIR
 fi
@@ -308,7 +308,7 @@ esac
 
 # Cleanup
 # ~~~~~~~
-unset archDir siteDir foundDir archOption
+unset archOption archDir siteDir _foamFoundDir
 unset gcc_version gccDir
 unset gmp_version gmpDir  mpfr_version mpfrDir  mpc_version mpcDir
 unset clang_version clangDir