[MacPorts] #56511: gcc49 @4.9.4_2: rev-upgrade rebuild fails on macOS 10.12.6 XCode 9.2

[MacPorts]

#56511: gcc49 @4.9.4_2: rev-upgrade rebuild fails on macOS 10.12.6 XCode 9.2
-----------------------+--------------------
  Reporter:  mopihopi  |      Owner:  (none)
      Type:  defect    |     Status:  new
  Priority:  Normal    |  Milestone:
 Component:  ports     |    Version:
Resolution:            |   Keywords:
      Port:  gcc49     |
-----------------------+--------------------

Comment (by kencu):

 I suppose the following test in the gcc <5 Portfiles should be an Xcode
 test rather than an os.major test:
 {{{
 if {${os.platform} eq "darwin" && ${os.major} > 16} {
     depends_lib
     depends_run
     archive_sites
     pre-fetch {
         ui_error "${name} is not supported on macOS High Sierra and newer"
         return -code error {unsupported platform}
     }
 }
 }}}

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Ticket URL: <https://trac.macports.org/ticket/56511#comment:2>
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