Specify -stdlib=libc++ if APPLE

If you specify any --target which refers to a x86_64-apple-darwin
triple, or a more specific variant derived from it, specifying
-stdlib=libc++ is required, otherwise clang falls back to libstdc++
which didn't include c++11, and fails to compile in very obscure ways
(simply failing to find any c++11 symbols).

Change-Id: I58025cea91eaa0c16d9b5831f9965889b75bbc31
Reviewed-on: https://boringssl-review.googlesource.com/21984
Reviewed-by: David Benjamin <davidben@google.com>
Commit-Queue: David Benjamin <davidben@google.com>
CQ-Verified: CQ bot account: commit-bot@chromium.org <commit-bot@chromium.org>

CMakeLists.txt

diff --git a/CMakeLists.txt b/CMakeLists.txt
index 1814b13..32f49a5 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -72,6 +72,9 @@
 
   if(NOT MSVC)
     set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -std=c++11")
+    if(APPLE)
+      set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -stdlib=libc++")
+    endif()
     if(NOT BORINGSSL_ALLOW_CXX_RUNTIME)
       set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -fno-exceptions -fno-rtti")
     endif()