Add --recurse-submodules to clone in git sources, so submodules are handled properly

[ipr-fredrik2]

Makes sure that submodules are handled properly when git package sources are cloned.

See #2807 2807

Very small PR, but untested as I don't have time to setup a build environment at the moment. Thus, this needs to be tested and verified. However this should just work(tm)

Imposes limit on git version >= 2.13

Please advise if you request further work from me, as I'd very much appreciate this (or an alternative solution) getting merged.

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