Add missing -std=c++11 flag when using non-appleclang compiler on macosx

[ajaypanyala]

When using non-appleclang compiler on macosx, i.e when using upstream (llvm.org) Clang,

if("${CMAKE_VERSION}" VERSION_GREATER 3.1.0)
  set(CMAKE_CXX_STANDARD 11)
  set(CMAKE_CXX_STANDARD_REQUIRED ON)
 ....

does not add -std=c++11 to the list of compile flags when building the Cpp runtime sources causing the build to fail. This happens when I am using any cmake version >= 3.1.

An alternative fix to the one suggested in this PR is to cmake_minimum_required (VERSION 3.0) instead of 2.8. Not sure why exactly, but the setting minimum version to 3.0 fixes the missing c++11 flag problem (i.e. I would not need to explicitly add the flag as done in this PR).

[mike-lischke]

OK, let's go with that explicit flag then. No need to raise the required CMake version. Please remove that min requirement line.

[ajaypanyala]

Removed the comment. Thanks!

[ajaypanyala]

@mike-lischke Could you please restart the one testcase that failed. From the build log, it's not clear why it fails.