BUG #6166: configure from source fails with 'This platform is not thread-safe.' but was actually /tmp perms

The following bug has been logged online:

Bug reference: 6166
Logged by: Alex Soto
Email address: apsoto(at)gmail(dot)com
PostgreSQL version: 9.0.4
Operating system: Linux (CentOS release 5.0 (Final))
Description: configure from source fails with 'This platform is not
thread-safe.' but was actually /tmp perms
Details:

I was trying to build the 9.0.4 source tarball as the postgres user on a
test machine.

The configure step failed with the error:
This platform is not thread-safe. Check the file 'config.log' or compile
and run src/test/thread/thread_test for the exact reason.
Use --disable-thread-safety to disable thread safety.

As I started looking through the log file I noticed that it failed to write
to the /tmp directory. Other configure steps had written to /var/tmp, but
this step tried to write to /tmp for some reason.

I fixed by correcting the permissions on the /tmp dir, but I thought the
error message was a little misleading, so I thought I'd report the problem.

The specific configure I ran was:
./configure --disable-integer-datetimes

but I don't think the config option likely makes a difference.