check_cxx_source_compiles("
#include <atomic>
#include <cstdint>
int main() {
   std::atomic<int> a1;
   int a1val = a1.load();
   (void)a1val;
   std::atomic<uint64_t> a2;
   uint64_t a2val = a2.load(std::memory_order_relaxed);
   (void)a2val;
   return 0;
}
" ROOT_HAVE_CXX_ATOMICS_WITHOUT_LIB)