ENH: rationalize wmake rules to avoid unnecessary duplication

- main mplib rules located in GENERAL_RULES with specializations
  (if any) under RULES. Modest change in a few options files, but
  cleans up the rules directories.

  previously in options:
      include $(RULES)/mplib$(WM_MPLIB)

  now in options:
      sinclude $(GENERAL_RULES)/mplib$(WM_MPLIB)
      sinclude $(RULES)/mplib$(WM_MPLIB)

  For GNU make it would be permissible to write both in a single line.