Use onbld mandoc.

*** 9,22 ****
  # at http://www.illumos.org/license/CDDL.
  #
  
  #
  # Copyright 2011, Richard Lowe
! # Copyright 2013 Nexenta Systems, Inc.  All rights reserved.
  #
  
! MANDOC=         $(ROOT)/usr/bin/mandoc
  ROOTMAN=        $(ROOT)/usr/share/man
  ROOTHASMAN=     $(ROOT)/usr/has/man
  FILEMODE=       0444
  
  # The manual section being built, client Makefiles must set this to, for e.g.
--- 9,22 ----
  # at http://www.illumos.org/license/CDDL.
  #
  
  #
  # Copyright 2011, Richard Lowe
! # Copyright 2014 Nexenta Systems, Inc.  All rights reserved.
  #
  
! MANDOC=         $(ONBLD_TOOLS)/bin/${MACH}/mandoc
  ROOTMAN=        $(ROOT)/usr/share/man
  ROOTHASMAN=     $(ROOT)/usr/has/man
  FILEMODE=       0444
  
  # The manual section being built, client Makefiles must set this to, for e.g.