scons: Build with C++11 with LLVM >= 3.5.

Starting with llvm-3.5svn r202574, LLVM expects C+11 mode.

commit f8bc17fadc8f170c1126328d203f0dab78960137
Author: Chandler Carruth <chandlerc@gmail.com>
Date:   Sat Mar 1 06:31:00 2014 +0000

    [C++11] Turn off compiler-based detection of R-value references, relying
    on the fact that we now build in C++11 mode with modern compilers. This
    should flush out any issues. If the build bots are happy with this, I'll
    GC all the code for coping without R-value references.

    git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@202574 91177308-0d34-0410-b5e6-96231b3b80d8

Signed-off-by: Vinson Lee <vlee@freedesktop.org>

scons/llvm.py

@@ -204,6 +204,7 @@ def generate(env):
             env.ParseConfig('llvm-config --ldflags')
             if llvm_version >= distutils.version.LooseVersion('3.5'):
                 env.ParseConfig('llvm-config --system-libs')
+                env.Append(CXXFLAGS = ['-std=c++11'])
         except OSError:
             print 'scons: llvm-config version %s failed' % llvm_version
             return