Support LLVM 3.5

LLVM 3.5 headers use C+11 and so CMake will not find them even
though they exist without passing -std=c++11 to its tests.

Signed-off-by: Edward O'Callaghan <eocallaghan@alterapraxis.com>
Merge pull request #241 from victoredwardocallaghan/llvm3.5

Support LLVM 3.5