[MacPorts] #54176: boost assumes libc++ is only used by clang

[MacPorts]

#54176: boost assumes libc++ is only used by clang
-------------------------+-------------------
 Reporter:  RJVB         |      Owner:
     Type:  enhancement  |     Status:  new
 Priority:  Normal       |  Milestone:
Component:  ports        |    Version:
 Keywords:  haspatch     |       Port:  boost
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 While testing the idea of using libc++ while building C++ code with G++ I
 ran into (amost) exactly the errors shown here :
 http://stackoverflow.com/questions/41050942/clang-modules-interaction-
 with-std-iterator-and-boost-move-iterator-hpp/43983903

 In this case the errors were not caused by libc++ but by the simple fact
 that in a few locations, boost assumes that libc++ is only used with
 clang.

 After fixing those presumptions with the attached patch I was able to
 build both boost and digiKam5 (which uses Boost and is a good test for any
 C++ compiler).

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Ticket URL: <https://trac.macports.org/ticket/54176>
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