[CMake] Prevent ExternalProject from updating git submodules

[Tim Rae]

Hi all,

I'm using ExternalProject_Add to add the popular C++ library range-v3 to 
one of our projects. However, since the build server only has local 
intranet access, and therefore can't access the main github repository, 
we have put a clone of that repo on our local gitlab:

ExternalProject_Add(range-v3
     PREFIX ${PREFIX_DIR}
     GIT_REPOSITORY ${LOCAL_GITLAB_URI}
)

The problem is that range-v3 has a documentation submodule that points 
to github, so `git clone ${LOCAL_GITLAB_URI} --recursive` would fail 
when github is inaccessible, whereas git clone works fine without the 
recursive flag.

I found a previous message from Tim Gallagher on the mailing list 
reporting the same problem, but it looks like the issue was never 
addressed upstream.

https://cmake.org/pipermail/cmake/2015-March/060100.html

Unfortunately, specifying GIT_SUBMODULES "" will add all submodules, so 
it doesn't appear that variable will help to solve the issue. I would 
suggest either having a special value like "__NONE__", or having a new 
boolean variable like INIT_GIT_SUBMODULES; either of which can be used 
to disable the automatic initialization of submodules entirely.

You could also change the default to NOT pull in submodules. This would 
be much better IMHO, as the current behavior is unintuitive. Most 
developers (e.g. myself until yesterday) don't even know what a 
submodule is, and are only familiar with using git clone, as opposed to 
git clone --recursive. Obviously this would be a breaking change though, 
so I understand that you probably don't want to go with this option.

For now I have "solved" the problem by adding a new commit to our 
range-v3 fork removing all git submodules, however this makes it more 
difficult to get the upstream changes in, so I highly recommend adding 
an option to disable the automatic initialization of git submodules.

Best regards,
Tim

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