TclMagick fails to load on Tcl8.6 due to the fix of https://sourceforge.net/tracker/index.php?func=detail&aid=1819422&group_id=10894&atid=310894
Attached a patch that fixes this issue by insuring that it is linked with libtclstub8.6.a
TkMagick may need something similar?
I still don't have Tcl 8.6 here to test with so I can not verify your patch. Sorry for taking so long!
This issue is addressed by Mercurial changeset 14967:e5f16218d00a