CONFIG: improve prefix matching for system libraries (#1607)

- missed detection of system libraries when installed with multiarch
  paths like /usr/lib/x86_64-linux-gnu

CONFIG: improve handling of group/user config files (#928)

- changed bashrc handling of FOAM_CONFIG_NOUSER to use
  FOAM_CONFIG_MODE instead. Propagate into foamEtcFile to make this
  a stickier control.

  This change allows better control, but also enables cluster
  installations to define their own value within the OpenFOAM prefs.sh
  file to prevent users accidentally mis-configuring things if
  necessary.

- remove undocumented handling of an (a)ll mode in foamEtcFile to
  avoid potential pitfalls.

- add support for FOAM_CONFIG_ETC handling.
  This allows injection of an extra search layer when finding
  project etc files

ENH: improvements to foamConfigurePaths (#928)

- handle FOAM_CONFIG_ETC implicitly, or explicitly with the new
  -etc option.

STYLE: more explicit wording in foamConfigurePaths usage (#1602)

- document that an absolute path (eg, -scotch-path) overrides/ignores
  the equivalent ThirdParty setting (eg, -scotch)

- longer options -system-compiler and -third-compiler for -system
  and -third, respectively. Clearer as to their purpose.

- adjust the location sanity check to look for META-INFO directory.

wmake/scripts/have_mgridgen

@@ -83,10 +83,10 @@ have_mgridgen()
     elif hasAbsdir "$prefix"
     then
         header=$(findFirstFile "$prefix/include/$incName")
-        library="$(findExtLib $libName $libName2)"
+        library=$(findExtLib "$libName" "$libName2")
     elif isSystem "$prefix"
     then
-        header=$(findFirstFile "/usr/local/include/$incName" "/usr/include/$incName")
+        header=$(findSystemInclude -name="$incName")
         prefix=$(sysPrefix "$header")
     else
         unset prefix
@@ -100,12 +100,10 @@ have_mgridgen()
     }
 
     # Library
-    [ -n "$library" ] || library=$(findLibrary \
-        "$prefix/lib/$libName" \
-        "$prefix/lib/$libName2" \
-        "$prefix/lib$WM_COMPILER_LIB_ARCH/$libName" \
-        "$prefix/lib$WM_COMPILER_LIB_ARCH/$libName2" \
-    ) || {
+    [ -n "$library" ] \
+    || library=$(findLibrary -prefix="$prefix" -name="$libName") \
+    || library=$(findLibrary -prefix="$prefix" -name="$libName2") \
+    || {
         #silent# [ -n "$warn" ] && echo "$warn (no library)"
         return 2
     }
@@ -165,7 +163,7 @@ have_mgridgen()
 }
 
 
-# Force reset of old variables
+# Reset variables
 no_mgridgen
 
 # Testing