Automatically detect export lists for OS X.

[EricWF]

Summary

Libc++ reexports symbols from the system libc++abi using -reexport_symbols_list. This can cause a linker failure if the list contains symbols not defined in the system libc++abi.
This patch attempts to detect the OS X version and use it to determine the correct symbol list.

It's my understanding that lib/libc++abi2.exp should be used on 10.9 and greater. Otherwise 'lib/libc++abi.exp' should be used

This fixes PR25666 (https://llvm.org/bugs/show_bug.cgi?id=25666)

[EricWF]

Accepting to commit.

lib/CMakeLists.txt

 
 # Setup flags.
 add_flags_if_supported(-fPIC)
 add_link_flags_if_supported(-nodefaultlibs)
 
 if ( APPLE AND (LIBCXX_CXX_ABI_LIBNAME STREQUAL "libcxxabi" OR
                 LIBCXX_CXX_ABI_LIBNAME STREQUAL "none"))
   if (NOT DEFINED LIBCXX_LIBCPPABI_VERSION)
+    set(LIBCXX_LIBCPPABI_VERSION "2") # Default value
+    execute_process(
+      COMMAND xcrun --show-sdk-version
+      OUTPUT_VARIABLE sdk_ver
+      RESULT_VARIABLE res
+      OUTPUT_STRIP_TRAILING_WHITESPACE)
+    if (res EQUAL 0)
+      message(STATUS "Found SDK version ${sdk_ver}")
+      string(REPLACE "10." "" sdk_ver "${sdk_ver}")
+      if (sdk_ver LESS 9)
+        set(LIBCXX_LIBCPPABI_VERSION "")
+      else()
         set(LIBCXX_LIBCPPABI_VERSION "2")
       endif()
+    endif()
+  endif()
 
   if ( CMAKE_OSX_DEPLOYMENT_TARGET STREQUAL "10.6" )
     add_definitions(-D__STRICT_ANSI__)
     add_link_flags(
       "-compatibility_version 1"
       "-current_version 1"
       "-install_name /usr/lib/libc++.1.dylib"
       "-Wl,-reexport_library,/usr/lib/libc++abi.dylib"