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/settings

@@ -96,7 +96,7 @@ case MSYS*:
     if ( "$WM_COMPILER" == Gcc ) setenv WM_COMPILER Mingw
     setenv WM_COMPILER_LIB_ARCH 64  # Consistent with linux64Mingw
     echo "openfoam: windows support (mingw64) is runtime only"
-    ;;
+    breaksw
 
 case SunOS*:
     setenv WM_ARCH solaris64
@@ -146,12 +146,12 @@ setenv FOAM_USER_APPBIN "$WM_PROJECT_USER_DIR/platforms/$WM_OPTIONS/bin"
 setenv FOAM_USER_LIBBIN "$WM_PROJECT_USER_DIR/platforms/$WM_OPTIONS/lib"
 
 # Prepend wmake to the path - not required for runtime-only environment
-set foundDir="${WM_PROJECT_DIR}/wmake"
+set _foamFoundDir="${WM_PROJECT_DIR}/wmake"
 if ( $?WM_DIR ) then
-    if ( -d "${WM_DIR}" ) set foundDir="${WM_DIR}"
+    if ( -d "${WM_DIR}" ) set _foamFoundDir="${WM_DIR}"
 endif
-if ( -d "$foundDir" ) then
-    setenv PATH "${foundDir}:${PATH}"
+if ( -d "$_foamFoundDir" ) then
+    setenv PATH "${_foamFoundDir}:${PATH}"
 else
     unsetenv WM_DIR
 endif
@@ -317,7 +317,7 @@ endsw
 
 # 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