USES=cmake now supports two additional list variables:
    * CMAKE_ON  : List of variables to turn on
    * CMAKE_OFF : List of variables to turn off
  This can be used as a shortcut to append these to CMAKE_ARGS.

  For example ports that previously set
    CMAKE_ARGS=		-DVAR1:BOOL=TRUE -DVAR2:BOOL=TRUE -DVAR3:BOOL=FALSE
  can now set this as
    CMAKE_ON=		VAR1 VAR2
    CMAKE_OFF=		VAR3

Reviewed by:	adridg, rakuco, mat
Differential Revision:	https://reviews.freebsd.org/D13636