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: