Buildfarm issues on specific machines

I spent a little time today cleaning up easily-fixed problems that are
causing buildfarm failures in various back branches. Hopefully that
will result in a few more "green" entries over the new few days. While
I was looking, I noticed several machines that seem to be failing
because of local issues:

potoroo [HEAD, 7.4]: lock file "/tmp/.s.PGSQL.65432.lock" already exists

I'm not sure if this is a problem with a stale lock file left around
from an old run, or if it happens because the machine is configured to
try to build/test several branches in parallel. In any case, it might
be worthwhile to try to hack the buildfarm script so that the Unix
socket files are allocated in a per-branch scratch directory, not in
/tmp. Or try to change pg_regress to use different port numbers for
different branches?

osprey [HEAD]: could not create shared memory segment: Cannot allocate memory
DETAIL: Failed system call was shmget(key=2, size=1957888, 03600).

Kernel shmem settings too small...

dragonfly [HEAD]: libz link error

As per earlier discussion, I maintain this is local misconfiguration.

cobra [7.4, 7.3, 7.2]: --with-tcl but no Tk

Possibly adding --without-tk to the configure options is the right answer.
Otherwise, install Tk or remove --with-tcl.

cuckoo [7.3, 7.2]: --enable-nls without OS support

This looks like pilot error; but the later branches don't fail on this
machine, so did we change something in this area?

caribou [7.2]: no "flex" installed

This looks like pilot error as well, though again I don't understand why the
later branches seem to work. Are we sure the same PATH is being used for
every branch here? Why doesn't the buildfarm report for 7.2 show the PATH?

regards, tom lane