[MacPorts] #67755: Build spdlog against Macports libfmt to avoid conflicts

[MacPorts]

#67755: Build spdlog against Macports libfmt to avoid conflicts
--------------------------+----------------------
 Reporter:  barracuda156  |      Owner:  judaew
     Type:  defect        |     Status:  assigned
 Priority:  Normal        |  Milestone:
Component:  ports         |    Version:  2.8.1
 Keywords:                |       Port:  spdlog
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 Could we build `spdlog` with `-DSPDLOG_FMT_EXTERNAL` option passed? And
 use our `libfmt10` (perhaps) or otherwise `libfmt9` port with it.

 Otherwise conflicts of headers arise, like here: https://github.com/oxen-
 io/oxen-logging/issues/10

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Ticket URL: <https://trac.macports.org/ticket/67755>
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