what is the path of kernel build directory?

[saeedeh motlagh]

hello guys
i want to install the openvswitch 1.4.0 from a linux package. the below
command should be executed:
./configure --with-linux=/lib/modules/'uname -r '/build
this is a linux command and i should execute the FreeBSD equivalent but i
don't know how to do that. the manual says:

   To build the Linux kernel module, so that you can run the
   kernel-based switch, pass the location of the kernel build
   directory on --with-linux.

what is kernel build directory in FreeBSD9 amd64? or how i should execute
this command?

yours,