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