FreeBSD Port: postgresql-contrib-9.0.0

[Yasu]

I did 'make install clean' at /usr/ports/databases/postgresql90-contrib/.
Then gmake show me the error message below:

>> pgbench.o(.text+0x320c): In function `main':
>> : undefined reference to `pthread_create'
>> gmake[1]: *** [pgbench] Error 1

Is this problem happened only on my FreeBSD8.1?
Could you tell me what should I do, please...
I watched the source, but it's difficult for me
 to understand it...

Best regards

-- 
Yasuaki Fukuda <stainmyheart at gmail.com>