cmake HAS_C11_ATOMICS examples

apple/foundationdb cmake/ConfigureCompiler.cmake :109 

CHECK_INCLUDE_FILE("stdatomic.h" HAS_C11_ATOMICS)

KhronosGroup/OpenCL-SDK samples/core/callback/CMakeLists.txt :37 

try_compile(
    HAS_C11_ATOMICS
    "${CMAKE_CURRENT_BINARY_DIR}"
    SOURCES "${CMAKE_CURRENT_BINARY_DIR}/atomic.c"
    C_STANDARD 11
    C_STANDARD_REQUIRED ON
)

KhronosGroup/OpenCL-SDK samples/core/callback/CMakeLists.txt :48 

elseif (NOT HAS_C11_ATOMICS)