[clang-warning] Don't pass -stdlib=libc++ on Apple platforms

Upstream clang has changed how it handles passing -stdlib=libc++ on
Apple platfoms. The rationale is that libc++ is the default standard
library, and stdlibc++ is no longer supported.

A  much more detailed rational can be found at
https://reviews.llvm.org/D139938.

Change-Id: I90f5fe8b63075b20e1632275c2751b6aa8a64800
Reviewed-on: https://boringssl-review.googlesource.com/c/boringssl/+/55928
Commit-Queue: Adam Langley <agl@google.com>
Auto-Submit: Paul Kirth <paulkirth@google.com>
Reviewed-by: Adam Langley <agl@google.com>
diff --git a/util/generate_build_files.py b/util/generate_build_files.py
index 34187bd..6c589f7 100644
--- a/util/generate_build_files.py
+++ b/util/generate_build_files.py
@@ -438,10 +438,6 @@
 
 if(CMAKE_COMPILER_IS_GNUCXX OR CLANG)
   set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -std=c++14 -fvisibility=hidden -fno-common -fno-exceptions -fno-rtti")
-  if(APPLE)
-    set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -stdlib=libc++")
-  endif()
-
   set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -fvisibility=hidden -fno-common -std=c11")
 endif()