cxxabi check fail on gcc4.8.5

[aengusjiang]

the newest version
cxxabi check fail on gcc4.8.5 centos7 os， but it support cxxabit

If I add the CXX standard at the root CMakeLists.txt， such as:
set(CMAKE_CXX_STANDARD 11)
set(CMAKE_CXX_STANDARD_REQUIRED ON)
it work ok

[jeremy-rifkin]

Thanks for the report, I'll look into fixing the check