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:

   * ./configure --enable-threads --enable-gcc

   * Edit Makefile:
        LIBS = $(MATH_LIBS) -pthread

   * make tcltest

   * Edit tclConfig.sh - linking of Tcl into static applications will need the -pthread switch to gcc
        TCL_LIBS=' -lm -pthread'

   * make

   * make install (as root, if necessary)

   * Rename tests/stack.test -> tests/stack.test_ to avoid known crash in test suite.

   * make test