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

[MacPorts]

#55471: Use separate portindexes for libc++ on older systems
--------------------------+----------------------------
  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 mojca):

 OK, it seems to work ok with
 {{{
 portindex -p macosx_12_libstdcxx_i386
 }}}
 but I guess the idea was to use just `macosx_12`. The problem is that in
 the `else` part of
 {{{
 if {[llength $platlist] > 3} {...}
 }}}
 the `cxx_stdlib` never gets set to its default value based on OS version,
 so it stays at `libc++` when the script is executed on >= 10.9.

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Ticket URL: <https://trac.macports.org/ticket/55471#comment:15>
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