Missing pip in sub-venv with parallel build

[stweil]

make all -j6 fails because pip is missing:

    Uninstalling pip-9.0.1:
    Uninstalling pip-9.0.1:
      Successfully uninstalled pip-9.0.1
Exception:
Traceback (most recent call last):
  File "/usr/lib/python3.6/shutil.py", line 544, in move
    os.rename(src, real_dst)
FileNotFoundError: [Errno 2] No such file or directory: '/root/ocrd_all/venv/local/sub-venv/headless-tf1/bin/pip' -> '/tmp/pip-07jk920k-uninstall/root/ocrd_all/venv/local/sub-venv/headless-tf1/bin/pip'

During handling of the above exception, another exception occurred:

It looks like one or two processes tried to upgrade pip at the same time.

[stweil]

It can be fixed by using the existing semaphores with $(SEM). PR will come soon.

[stweil]

This was fixed with pull request #166.