Update build steps to work with arbitrary commits and updated submodules

[jonaseberle]

Checking out a new commit or switching branches that update submodules requires additional steps to checkout the submodules to that commit.

git submodule update --init --recursive takes care of that (while the current README only works for fresh clones).

The second change is that mkdir -p will succeed even if the directory is already existent, thus making the whole recipe easier to just use as copy-paste.

[jonaseberle]

(tested with git version 2.38.1, but I don't see why it should not work even in older git. In fact our previous --recurse-submodules required 2.13 I think)