[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:
      Type:  enhancement  |     Status:  new
  Priority:  Normal       |  Milestone:
 Component:  base         |    Version:
Resolution:               |   Keywords:
      Port:               |
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Comment (by mojca):

 I'm sorry, I totally missed the patches. You could potentially open a Pull
 request against base to simplify some further testing, but it would be
 awesome if these patches were actually deployed, soon.

 Regarding the usage of variable in the port: in the past (before we had
 cxx11 1.1) I would switch to libc++ for some ports just because that would
 allow me to compile them and if they had no dependency on another C++11
 api, this was never a problem. I found it weird that I had to duplicate
 the code in the PortGroup just for the sake of compiler blacklisting, but
 if I only used the cxx 1.0 PortGroup, the port would not compile at all,
 so I had to duplicate the code anyway. At some point I found a workaround
 to first change `configure.cxx_stdlib` and then include the PortGroup as
 that would trick the PortGroup into believing everything was fine. But I
 could no longer distinguish whether it was the port that set that variable
 or if it was set globally. I forgot all the details and I'm not willing to
 go back to history (this was the case with root6 for sure), but just to
 give a rough idea.

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Ticket URL: <https://trac.macports.org/ticket/55471#comment:6>
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