[CMake] Prevent ExternalProject from updating git submodules

[Tim Gallagher]

Hi everybody,

I'm plugging away at using the ExternalProject features to package up our code. Looking at the documentation for ExternalProject_Add, it says:

...
[GIT_SUBMODULES modules...] # Git submodules that shall be updated, all if empty
...

which is cool, but what do I do if I don't want any submodules updated at all? It seems my only choices are:

1. Update them all (leaving GIT_SUBMODULES empty)
2. Update some of them (set GIT_SUBMODULES to a list to update)

but there's no 3rd option of not updating them at all. 

Any advice on how to keep it from updating any submodules?

Thanks,

Tim