ENH: improve handling of pt-scotch headers/libraries

- provide dedicated detection 'have_ptscotch' function that can be
  used after the 'have_scotch' function.

  It sets the PTSCOTCH_ARCH_PATH, PTSCOTCH_INC_DIR, PTSCOTCH_LIB_DIR
  and helps when the serial and parallel versions are located with
  different logic.

wmake/scripts/sysFunctions

@@ -48,14 +48,20 @@ then
     esac
 
 
-    # True if OS is Darwin.
-    # Uses libso extension to cache the value
-    # (instead of calling 'uname -s' each time)
+    # True if target OS is Darwin.
+    # Uses cached value from libso extension
     isDarwin()
     {
         test "$extLibso" = ".dylib"
     }
 
+    # True if target OS is Windows
+    # Uses cached value from libso extension
+    isWindows()
+    {
+        test "$extLibso" = ".dll"
+    }
+
 
     # True if '$1' begins with '/'
     isAbsdir()