As discussed in the section on differences between the C source and the Tcl source code, Tcl has no pre-processor or native ability to define constants.
I have added the ability to define constants based on this source code.
I have also implemented all Future Lab Tcl software by placing constants that are required by more than one module into what I call definition files. Most definition files contain only constants with the exception of the main Future Lab definition file stdhead.tcl.
This main definition file also contains logic to check for the presence of the mandatory TCLDIR directory tree variable (as discussed here). This main definition file also assigns the switch character used in command line arguments and sources other Tcl modules.