[MacPorts] #71395: libsigcxx2/3, libxmlxx5: Reduce dependencies

[MacPorts]

#71395: libsigcxx2/3, libxmlxx5: Reduce dependencies
--------------------------+-----------------------------------------------
 Reporter:  Dave-Allured  |      Owner:  dbevans
     Type:  enhancement   |     Status:  assigned
 Priority:  Normal        |  Milestone:
Component:  ports         |    Version:
 Keywords:                |       Port:  libsigcxx2, libsigcxx3, libxmlxx5
--------------------------+-----------------------------------------------
 Dependency `graphviz` can probably be removed for these three ports,
 thereby removing a heavy load.  Upstream changes were made recently,
 making the `dot` command from `graphviz` optional for documentation.
 Upstream discussions and commits:

 https://github.com/libsigcplusplus/libsigcplusplus/issues/98

 https://github.com/libxmlplusplus/libxmlplusplus/commit/74608b97a3a6a9b230e28c5abaa138160c708838

 Because of a parallel dependency chain, reductions will not be realized
 until [https://trac.macports.org/ticket/71389 trac 71389] for `libproxy`
 is completed.

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Ticket URL: <https://trac.macports.org/ticket/71395>
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