Lets exchange here our solutions to finding out tclConfig.sh.
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'''[DGP]''' Let's put the correct solution first:

  package require Tcl 8.5
  set d [::tcl::pkgconfig get libdir,install]
  puts [file join $d tclConfig.sh]
  exit

For the large number of people not yet using Tcl 8.5, continue...
----


'''[Googie]''' My first candidate is following code:

 foreach d [concat \
     [list $tcl_library \
           [lindex $tcl_pkgPath 0]] \
     $auto_path \
     [list [file dirname $tcl_library] \
           [file dirname [lindex $tcl_pkgPath 0]] \
           [file dirname [file dirname $tcl_library]] \
           [file dirname [file dirname [lindex $tcl_pkgPath 0]]] \
           [file dirname [file dirname [file dirname $tcl_library]]] \
           [file dirname [file dirname [file dirname [lindex $tcl_pkgPath 0]]]]] \
 ] {
     if {[file exists [file join $d tclConfig.sh]]} {
         puts "[file join $d tclConfig.sh]"
         exit
     }
 }

 puts "none"

We can put it into the one tcl script file and execute from a shell to get info, if we can find tclConfig.sh (returns path pointing to it) or we can't (returns "none"). I use few levels of [file dirname] for $tcl_library and $tcl_pkgPath, because of some problems on MacOS (tclConfig.sh is places few levels upper than $tcl_library).