[CMake] add_subdirectory namespaces

[Robert Maynard]

Subsequent calls to project() from sub-directories is supported. For
calls to project from sub-directories it does everything but set
CMAKE_PROJECT_NAME.

On Wed, Jul 24, 2019 at 7:29 AM Jason Beach <jason.m.beach at gmail.com> wrote:
>
> I've been reorganizing / updating our software cmake build. It currently uses a mixture of Externa Projects and normal target definitions that depend on the external projects (which has you probably know causes much sorrow and grief).  One of my goals in the reorganization was to replace all the ExternalProjectAdd statements with FetchContent, however I soon discovered that some of the external projects declare targets with the same name (i.e. an uninstall target).  These external projects are third party libraries that are not under our control.
>
> It appears there's been a suggestion to add namespaces to the add_subdirectory command:
> https://gitlab.kitware.com/cmake/cmake/issues/16414
>
> Is that going anywhere?  It seems like that would address FetchContent's main limitation. I guess for the time being I'll use ExternalProject for thirdparty libraries not under our control and FetchContent for libraries that are and do a true super build instead of the muddled sort of arrangement we have now.
>
> One other kind of not-so-random question-the documentation on the Project statement is fine if it's in the toplevel CMakeLists.txt (in which case it defines some project level variables, etc.) but it's silent about what happens if it's not in the top level CMakeLists.txt.  i.e. if I add a library via add_subdirectory or FetchContent and that library's toplevel CMakeLists.txt has the project statement, what happens when it's executed?  Is it silently ignored?
>
> Thanks,
> Jason
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