[CMake] Specifying a pre-build dependency?

[Bryan O'Sullivan]

I'd like to set up a scheme in which I install a third-party SDK on 
demand, before compiling the code that depends on it.  In principle, 
this is something that a pre_build target would help with, but that's 
VS-specific.

Is there some other non-cumbersome approach I could use, short of 
something kludgy like ADD_CUSTOM_TARGET that will always be run?

	<b