Make csources a submodule

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.

1 files changed, 0 insertions, 0 deletions

diff --git a/csources b/csources
new file mode 160000
+Subproject 15724e2e1f3e7749d508dfcd995e84fea285080