[Cmake] Substitution or not between quotes?

[Brad King]

Martin Trudeau wrote:
> could you give me an example
> where quotes in the first argument of an IF with ${...}present 
> have indeed an effect?

SET(x y z)
IF("${x}")
ENDIF("${x}")

Without the double quotes, the expansion produces

IF(y z)
ENDIF(y z)

which is not a valid IF command.

 > And what do you do when you want
> to match a string to a regular expression
> 
> IF ("some_string" MATCHES regex)
> 
> but are not sure some_string isn't already defined?
> Do you absolutely have to test it first with IF(DEFINED...)?

Yes.  I agree this is strange behavior but there are historical and 
backward compatibility reasons for it.

-Brad