* Fix #444 - Use BENCHMARK_HAS_CXX11 over __cplusplus.

MSVC incorrectly defines __cplusplus to report C++03, despite the compiler
actually providing C++11 or greater. Therefore we have to detect C++11 differently
for MSVC. This patch uses `_MSVC_LANG` which has been defined since
Visual Studio 2015 Update 3; which should be sufficient for detecting C++11.

Secondly this patch changes over most usages of __cplusplus >= 201103L to
check BENCHMARK_HAS_CXX11 instead.

* remove redunant comment