[CMake] how to use DEFINED with ENV

[Brad King]

Xavier Delannoy wrote:
> my CMakeLists.txt need an environement variable. 
> 
> I try this, with no success.
> /tmp> cat CMakeLists.txt 
> IF(DEFINED ENV{TEST})
>         MESSAGE("test is defined")
> ELSE(DEFINED ENV{TEST})
>         MESSAGE("test is not defined")
> ENDIF(DEFINED ENV{TEST})

I've implemented support for a test of this form.  It will be included 
in the next release.

-Brad