enable thready safety on Mac OS X 10.3.4

I attempted to compile 7.4.3 with enable-thread-safety but it fails and
says it's not supported. My configure command was:

./configure --enable-thread-safety

and the result:

checking pthread.h usability... yes
checking pthread.h presence... yes
checking for pthread.h... yes
configure: error:
Cannot enable threads on your platform.
Please report your platform threading info to the PostgreSQL mailing
lists
so it can be added to the next release. Report all compile flags, link
flags,
functions, or libraries required for threading support.
See the comment at the top of src/port/thread.c for more information.

This is on OS X 10.3.4 with the latest developer tools. Am I doing
something wrong, or should this be reported as a bug?

Thanks,
----
Aaron Burghardt
aburgh(at)mac(dot)com