cmake BOOST_COBALT_HAS_STD_PMR examples

Jackarain/proxy third_party/boost/libs/cobalt/cmake/CheckRequirements.cmake :49 

try_compile(BOOST_COBALT_HAS_STD_PMR
        ${CMAKE_CURRENT_BINARY_DIR}
        ${CMAKE_CURRENT_LIST_DIR}/memory_resource.cpp
        CXX_STANDARD 17
        CXX_STANDARD_REQUIRED 17)