Circular dependency with LDC and Phobos

[Pjotr Prins]

We are trying to build the LLVM D-compiler (ldc). The problem is a
circular dependency on a D-library (Phobos). The D-compiler source
won't compile without the Phobos sources which are in a separate
repository. Normally it is handled by git --recurse, but I think it
would be better to download the tar balls of the releases (I think git
recurse submodules is not working in Guix anyway, last time I tried).

How best to deal with this? Should I write a package which unpacks
Phobos tarball and make the source available to ldc? ldc won't build
without Phobos and Phobos won't build without ldc. 

>From the source:

LDC currently needs custom forks of druntime and Phobos. They are
integrated with the main repository using Git submodules. To build
them, make sure you have up-to-date copies of the submodules in your
local repository:

$ git submodule update -i