Remove spurious comment
Fix typos
Support concurrent preparation of distribution archives

The preparation of distribution archives uses a symbolic link to
${SRCDIR} whose creation and removal creates a race condition for
concurrent preparation of distribution archives.  We reorganise code
to avoid this race condition.

We also ensure that GPG signatures are prepared in sequence, which is
required by the password interaction involved in the signature
process.
Run tests in sequence

The interactions between parallel mode and the use of distinct objdirs
seems to puzzle make.  We therefore require that tests are performed
in sequence.  Parallel mode is preserved within tests.