I just noticed that my build of Ruby 1.8 does not include tcltklib. 
Running the extconf.rb file in ext/tcltklib manually gives ...

$ ruby extconf.rb
checking for t_open() in -lnsl... no
checking for socket() in -lsocket... no
checking for dlopen() in -ldl... yes
checking for log() in -lm... yes
checking for tcl.h... no

Although it is not finding it, I do have a tcl.h file in 
/usr/include/tcl8.3.

I am running Debian Linux and have both the tk8.3-dev and tcl8.3-dev
packages installed.

Any ideas?

-- 
-- Jim Weirich       jweirich / one.net      http://onestepback.org
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"Beware of bugs in the above code; I have only proved it correct, 
not tried it." -- Donald Knuth (in a memo to Peter van Emde Boas)