[CMake] $ENV usage and documentation

[Bill Hoffman]

1. $ENV{VAR} is supported to get environment variables.

2. You can set them with SET(ENV{VAR} value), however, it is only set as 
long as CMake is running.  Really only useful in ctest scripts or 
scripts that need to pass information to programs they directly run.

3. If something is missing from the documentation, the best way to make 
sure it gets fixed is to create a bug entry.  I realize there are lots 
of bugs not yet fixed.   However, it is far more likely to be lost in 
the mailing list.  The bug tracker can be monitored and "pinged".  The 
bugs that are "easiest" to fix are the ones that get fixed fastest.  So, 
a small focused patch for documentation will be addressed much quicker 
than a large complicated patch.

Thanks.

-Bill