1. Solves a problem that updating Nim sources we have no indication we need to
update csources as well.
2. Using submodule we can explicitly request that some Nim version requires
some csources version to bootstrap.
3. build.sh now simply calls: git submodule update --init --depth 1
instead shallow clone on missing csources subdirectory.
I am not sure why this is not set already? To me it is an annoyance
that I have to set it manually and then git shows me a change for
that file all the time while working.
