[MacPorts] #55471: Use separate portindexes for libc++ on older systems

[MacPorts]

#55471: Use separate portindexes for libc++ on older systems
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  Reporter:  ryandesign   |      Owner:  neverpanic
      Type:  enhancement  |     Status:  assigned
  Priority:  Normal       |  Milestone:  MacPorts 2.5.0
 Component:  base         |    Version:
Resolution:               |   Keywords:
      Port:               |
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Comment (by ryandesign):

 Replying to [comment:11 mojca]:
 > {{{
 > if {${cxx_stdlib} eq "libc++"} {
 > }}}

 Test `${configure.cxx_stdlib}` instead. `${cxx_stdlib}` is the value in
 macports.conf and does not represent the possibly-modified value the port
 will actually build with.

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Ticket URL: <https://trac.macports.org/ticket/55471#comment:13>
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