[CMake] test for environment variable?

[David Somers]

> You can find the value of an environment variable with $ENV{VAR}.

I know.

> I don't think there is a way to tell the difference between being set to
> nothing and not being set at all.

Which is what I was afraid of. I had previously tried something along the
lines of IF (DEFINED $ENV{BLAH}) without success

Guess I'd better add a request for this :-)

Cheers,

David