[CMake] Testing whether a variable has already been assigned a non-null value

[Brad King]

Alan W. Irwin wrote:
> I have noticed tests like
> 
> IF("${VARIABLE}" MATCHES "^${VARIABLE}$")
> 
> in, e.g., CheckFunctionExists.cmake where VARIABLE is a macro argument.
> From
> the context I assume it is a test for whether the actual variable used in
> the macro call has been assigned a non-null value, but I don't get why
> it works. Could somebody please explain?
> 
> What is the best way to decide if the variable X has been assigned a
> non-null value when X is not an argument of a macro?  Something like
> "IF(NOT
> X)" ?  Or is there some transformation of the above MATCHES text that is
> recommended?

IF(DEFINED X)

The MATCHES hack is left over from code written before IF(DEFINED ...)
was implemented.

-Brad