ENH: robuster handling of system or central installation (issue #290)

- Follow similar pattern as per boost/CGAL with the '*-none' to disable,
  and '*-system' for system installations.

  Also support central non-ThirdParty installations by specifying the
  version as "*-system", but also providing the fully qualified
  *_ARCH_PATH too.

--
ENH: Verify label and scalar sizes used in the metis header.

- ensure they match with expected values, which can be obtained via
  the OpenFOAM definitions for WM_LABEL_SIZE and WM_PRECISION_OPTION

etc/config.sh/metis

@@ -28,13 +28,33 @@
 #     Setup file for metis include/libraries.
 #     Sourced during wmake process only.
 #
+#     Normally used to specify the metis version and location for a
+#     ThirdParty installation.
+#
+#     If using system-wide installations, use the following setting:
+#
+#         METIS_VERSION=metis-system
+#
+#     If the system metis is unusable (eg, too old) and you don't
+#     have or want a ThirdParty installation:
+#
+#         METIS_VERSION=metis-none
+#
+#     If using a central installation, but not located under ThirdParty:
+#         - specify metis-system
+#         - provide full paths for METIS_ARCH_PATH
+#
 # Note
 #     A csh version is not needed, since the values here are only sourced
-#     during the wmake process
+#     during the wmake process.
+#
+#     Metis can also be entirely disabled, by either renaming this file or
+#     by creating an empty one with the same name at a user or site location.
 #
 #------------------------------------------------------------------------------
 
-export METIS_VERSION=metis-5.1.0
+METIS_VERSION=metis-5.1.0
+
 export METIS_ARCH_PATH=$WM_THIRD_PARTY_DIR/platforms/$WM_ARCH$WM_COMPILER$WM_PRECISION_OPTION$WM_LABEL_OPTION/$METIS_VERSION
 
 #------------------------------------------------------------------------------