[MacPorts] #49903: libcxx: undo changes to system directories when deactivating

[MacPorts]

#49903: libcxx: undo changes to system directories when deactivating
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  Reporter:  ryandesign   |      Owner:  jeremyhu
      Type:  enhancement  |     Status:  new
  Priority:  Normal       |  Milestone:
 Component:  ports        |    Version:  2.3.4
Resolution:               |   Keywords:
      Port:  libcxx       |
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Comment (by ryandesign):

 [comment:ticket:55441:10 Mojca pointed out] that a user might install two
 copies of MacPorts, install libcxx in each of them, and then later
 uninstall libcxx in one of them. In that scenario, it's actually desirable
 that libcxx doesn't clean up after itself after it's uninstalled,
 otherwise the libc++-requiring ports in the remaining MacPorts
 installation would be hosed.

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Ticket URL: <https://trac.macports.org/ticket/49903#comment:9>
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