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
----
[LV] why not submit a bug report to http://tcl.sf.net/ and have these
fixes moved into the configure script for all ?
----
Filing a formal bug report is a good idea, and I will do that.

Jeff and I spent some time trying to fix this for the Tcl 8.3.4
release, but the apparent right fix for this on FreeBSD broke the
build/test on AIX.  Neither of us was willing to break something
that was working for a patch release, so the backup solution was
to post the FreeBSD work-around here.  Since threaded builds on
FreeBSD have never worked "out of the box" yet, we haven't lost anything.