Since acceptance-tests/Makefile (and llvm/Makefile) include scripts/submodules/versions.mk which evals versions.py which looks
for a SUBMODULES.json file in the current working directory we got confusing error messages if the build directory is not the src dir:
```
Traceback (most recent call last):
File "../../mono/scripts/submodules/versions.py", line 22, in <module>
submodules = json.load(open(CONFIG_FILE))
IOError: [Errno 2] No such file or directory: 'SUBMODULES.json'
```
Instead we now pass the path to SUBMODULES.json as a variable to versions.py and set it based on $(top_srcdir) so it works
even for out-of-tree builds.
It got broken when the versions.py file was moved around in d110f526706b7c4880ca916e2dfbab93138871cd.
Also add the new locations to EXTRA_DIST so they get distributed too.
* Move the versions.mk infrastructure to scripts/submodules so it can be used elsewhere too.
* Add a conditional submodule for llvm.
* Fix commit messages emitted by the bump- targets.
* Use a anonymous git url for llvm.
