Incorporate external code in the form of git clones

[andrey-popov]

Currently I simply keep a copy of the original code in external/JEC. But it seems one can add a clone of a repository directly, making it pointing to a specific commit. See an example here.

[andrey-popov]

Building of external code is switched off as of commit e9a89c4. Enabling it back might require changes in directory structure of this code, and thus might conflict with this issue.

[andrey-popov]

Or, maybe, even move the code for JERC into an independent repository.