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

[MacPorts]

#44129: cmake: fix build against libc++ on Lion
----------------------+-------------------------
  Reporter:  mojca@…  |      Owner:  michaelld@…
      Type:  defect   |     Status:  new
  Priority:  Normal   |  Milestone:
 Component:  ports    |    Version:
Resolution:           |   Keywords:  lion
      Port:  cmake    |
----------------------+-------------------------

Comment (by mojca@…):

 Michael, are you willing to commit the change that you suggested in
 comment:27 or do you want me to do any further testing?

 (I guess you don't need the fallback because `macports-clang-3.4` is
 already on the list of defaults, but maybe double-check in the MacPorts
 sources just in case. It certainly is on the default fallback list on 10.7
 and on < 10.7 you cannot use `libc++` without clang 3.4 to begin with. But
 if you want to add clang 3.5, you could add all newer versions as well
 then.)

-- 
Ticket URL: <https://trac.macports.org/ticket/44129#comment:29>
MacPorts <https://www.macports.org/>
Ports system for OS X