This is a judgement call on Redhat's part, but there are some problems with compiling tcl against pthreads. To wit, with glibc2.1 (according to andreas jaeger) it is impossible to load a shared object which is linked against pthreads. A bit of background: I am the author of an Apache module, mod_dtcl, which is a module for doing server-parsed Tcl, similar to PHP. I had received numerous bug reports that there were problems with this module on redhat boxes. So, as you can see, it's a problem if you wish to use this Apache module as a shared object, as mod_dtcl.so is linked against tcl, which is in turn linked against pthreads on redhat boxes. Comments? Questions? Thanks!
Thanks for the comment, as I turned on --enable-threads in order to get feedback. I will revert the change in Raw Hide, as it seems the linux world is not quite ready for --enable-threads in tcltk. Please reopen this bug when that time comes :-)