build: use libc++ on OSX

V8 now depends on C++11 runtime features. On OSX this requires us to
link against the libc++ library rather than the deprecated default
that is provided with -mmacosx-version-min=10.7.

PR-URL: https://github.com/nodejs/node/pull/8317
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Michaël Zasso <targos@protonmail.com>

common.gypi

@@ -385,6 +385,7 @@
             'xcode_settings': {
               'GCC_VERSION': 'com.apple.compilers.llvm.clang.1_0',
               'CLANG_CXX_LANGUAGE_STANDARD': 'gnu++0x',  # -std=gnu++0x
+              'CLANG_CXX_LIBRARY': 'libc++',
             },
           }],
         ],