[CMake] environment flag for cmake

[Brad King]

Shaul Kedem wrote:
>  What I want is the option to create only a subset of a project which
> uses more than one cmake lists. The idea we came up with over here was
> to comment out the part which include other cmake lists when we want a
> subset. if there is another way I'd very much like to hear about it.

OPTION(EXTRA_STUFF "Build extra portion" OFF)
IF(EXTRA_STUFF)
   SUBDIRS(extra_stuff)
ENDIF(EXTRA_STUFF)

-Brad