[MacPorts] #59584: macports-clang versions < 5.0 don't understand -stdlib=macports-libstdc++

[MacPorts]

#59584: macports-clang versions < 5.0 don't understand -stdlib=macports-libstdc++
--------------------+--------------------
 Reporter:  kencu   |      Owner:  (none)
     Type:  defect  |     Status:  new
 Priority:  Normal  |  Milestone:
Component:  base    |    Version:  2.6.2
 Keywords:          |       Port:
--------------------+--------------------
 currently base will try to pass `-stdlib=macports-libstdc++` to clang 3.4
 or 3.7 under certain circumstances of blacklisting and whitelisting when
 the  compiler c or c++ standards are set to >= 2011.

 in  [https://github.com/macports/macports-
 base/blob/aeb6d4386cd92afd0924a69bc8ee6154e651e8b3/src/port1.0/portconfigure.tcl#L810
 portconfigure:get_min_clang] the logic to select a proper clang doesn't
 take into account the fact that macports-clang versions < 3.9 won't be
 able to understand `-stdlib=macports-libstdc++`.

 we don't have `macports-clang-3.9` any more, so practically for us
 macports-clang versions < 5.0 can only be used for compiler.cxx_standard
 >= 2011 or compiler.c_standard >= 2011 if `configure.cxx_stdlib=libc++`.

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Ticket URL: <https://trac.macports.org/ticket/59584>
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