Parallel make: additional fixes

Parallel make: additional fixes

Testing turned up a few more errors with parallel builds:

- Linux: Apply a similar fix for "make dest" to the one in the previous
patch for "make install".  Avoid conflict between DESTDIRS and COMPDIRS,
which also prevents unneeded duplicate work.

- Fix a couple of cases where we have 2 targets in a single rule with a
command that generates both of them.  Two parallel jobs can run this
command at the same time, with missing or corrrupted files as a result.
The workaround: make one of the targets build the files, and make the
second target depend on the other one.

Reviewed-on: http://gerrit.openafs.org/57
Verified-by: Russ Allbery <rra@stanford.edu>
Reviewed-by: Russ Allbery <rra@stanford.edu>
Reviewed-by: Derrick Brashear <shadow@dementia.org>