[MacPorts] #44129: cmake: fix build against libc++ on Lion

[MacPorts]

#44129: cmake: fix build against libc++ on Lion
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  Reporter:  mojca@…  |      Owner:  css@…
      Type:  defect   |     Status:  new
  Priority:  Normal   |  Milestone:
 Component:  ports    |    Version:
Resolution:           |   Keywords:  lion
      Port:  cmake    |
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Comment (by jeremyhu@…):

 Good find.  I don't see anything obviously releated in istream when
 comparing the STL (/opt/local/libexec/llvm-3.[34]/lib/c++/v1/istream), but
 if you figure out the cause, we can rev clang-3.3 with the fix.

 Note that macports-clang-3.4 should already be in the fallback list, so
 you really just need the compiler.blacklist-append line.

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Ticket URL: <https://trac.macports.org/ticket/44129#comment:5>
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