[CMake] Difference between IMPORT/EXPORT and External Project

[Nathaniel Waisbrot (Cont ARL/CISD)]

> I'm trying to figure out in what scenarios one might use import/export
> verses using the new ExternalProject_add.
...
> 
> When would one pick one method over the other?


I haven't used import/export, and don't know about them.  I am using
ExternalProject to build self-contained, wholly-separate sub-projects
that are a prerequisite of my project.  I will attempt to explain how I
use it:

If you are familiar with Subversion's svn:externals, it is the same
idea: not managed by your project, but needed by your project.

An example: you've created a project that makes use of the Foo
pre-processor, an obscure little open-source program.  Rather than
remembering to check for updates and install the FPP by hand, you can
define it as an ExternalProject that's updated and built as a
prerequisite to the parts of your project that need it.