Add a generator expression for immediate-if
I've wanted to get my feet wet with CMake development for a while, so when I spotted this feature request tagged as "easy", and corresponding to a feature I've wanted for a while, I decided to give it a try. Here's the result of that.
This merge defines a new generator expression $<IIF>
(named after the immediate-if function). It takes three parameters: a conditional, a truth-value, and a false-value. If the conditional is 1
, the expression evaluates to the truth-value. If the conditional is 0
, the expression evaluates to the false-value.