cmake BOOST_LOG_HAS_XOPEN_SOURCE_600 examples

Jackarain/proxy third_party/boost/libs/log/CMakeLists.txt :92

check_cxx_source_compiles("#include <${CMAKE_CURRENT_SOURCE_DIR}/config/xopen-source-600/xopen_source_600.cpp>" BOOST_LOG_HAS_XOPEN_SOURCE_600)