CXXFeatures: add support for XL

Information taken from:
https://www.ibm.com/developerworks/community/blogs/5894415f-be62-4bc0-81c5-3956e82276f3/entry/xlc_compiler_s_c_11_support50?lang=en
Merge topic 'cxx11' into next

a85c500 CXXFeatures: add support for XL