[MacPorts] #37583: Update tcl.m4 macro in base to add a check for tcl8.6

[MacPorts]

#37583: Update tcl.m4 macro in base to add a check for tcl8.6
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 Reporter:  egall@…      |      Owner:  macports-tickets@…
     Type:  enhancement  |     Status:  new
 Priority:  Normal       |  Milestone:
Component:  base         |    Version:  2.1.2
 Keywords:  haspatch     |       Port:
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 Pretty simple patch, just adds a few lines, it's attached

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Ticket URL: <https://trac.macports.org/ticket/37583>
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