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.

.gitmodules

@@ -0,0 +1,3 @@
+[submodule "csources"]
+	path = csources
+	url = ../../nim-lang/csources.git