I don't see the --without-pthreads option documented anywhere in
/courier/docs nor INSTALL. Not even in the .configure --help screen.
This is one of the things that normally aren't necessary, and exist solely
to enable bypass of unintended breakage. I got a note from Brian Candler
about the weird way FreeBSD implements thread support, so I tried to change
the configuration script for that. I'm waiting for an update.