For an ordinary build/install of Tcl 8.3.4 on FreeBSD, the standard instructions should work just fine:
./configure make su (if needed) make install exit (the su) make test (if desired)
For a thread-enabled build, though:
./configure --enable-threads ... configure: warning: Don t know how to find pthread lib on your system - you must disable thread support or edit the LIBS in the Makefile... ...
So, here's the recipe, involving some file editing, for building, installing and testing a thread-enabled Tcl on FreeBSD: