[CMake] add_custom_command as target

[Gonzalo Garramuño]

Brandon Van Every wrote:
> 
> Admittedly this is inelegant, but it works.  I have no idea how much
> work it would be to remove the distinction between file level and
> target level dependencies.
> 

It isn't inelegant, but quite logical.  It probably should be kept as 
is, as it allows easily adding multiple dependencies to a single target.

I thought of trying that as a last resort but by then I had more or less 
given up.

That feature is, however, undocumented.  Neither of the add_custom_* nor 
the wiki has any mention of this or the possibility of add_custom_target 
being able to take a custom command.

-- 
Gonzalo Garramuño
ggarra at advancedsl.com.ar

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