[MacPorts] #65247: OpenVPN3 build fails on older OS versions

[MacPorts]

#65247: OpenVPN3 build fails on older OS versions
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  Reporter:  iAmInActions   |      Owner:  i0ntempest
      Type:  defect         |     Status:  assigned
  Priority:  Normal         |  Milestone:
 Component:  ports          |    Version:  2.7.2
Resolution:                 |   Keywords:
      Port:  openvpn3 asio  |
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Comment (by jmroot):

 The comments in `AssertMacros.h` actually discuss exactly this kind of
 problem, and there is a mechanism to avoid it: simply define
 `__ASSERT_MACROS_DEFINE_VERSIONS_WITHOUT_UNDERSCORES` to 0 before the
 header is included. It's not immediately clear where it's being pulled in,
 however—could be in openvpn3, or asio, or somewhere else?

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Ticket URL: <https://trac.macports.org/ticket/65247#comment:6>
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