[CMake] elseif request

[Ken Martin]

So ...

> if(cond1)
>    block of statements
> elseif(cond2)
>    block of statements
> elseif(cond3)
>    block of statements
> 
> ...
> 
> elseif(condn)
>    block of statements
> else(cond1)
>    block of statements
> endif(cond1)

And with LOOSE set

> if(cond1)
>    block of statements
> elseif(cond2)
>    block of statements
> elseif(cond3)
>    block of statements
> 
> ...
> 
> elseif(condn)
>    block of statements
> else()
>    block of statements
> endif()

Seems like a reasonable idea to me and I think it can be implemented fairly
easily. Folks who like more strict closing structures can nest their IF
statements as is currently done. 

Ken