Fix compile errors when Tcl threads aren't available

[uatuko]

Close #19

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.64%. Comparing base (229071a) to head (9b4c57c).
Report is 5 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master      #21   +/-   ##
=======================================
  Coverage   98.64%   98.64%           
=======================================
  Files           3        3           
  Lines          74       74           
=======================================
  Hits           73       73           
  Misses          1        1           

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