cmake FLB_HAVE_STDATOMIC_H examples

fluent/fluent-bit CMakeLists.txt :749 

check_c_source_compiles("
       #include <stdatomic.h>
       int main() {
          _Atomic int a;
          return 0;
       }" FLB_HAVE_STDATOMIC_H)

fluent/fluent-bit CMakeLists.txt :756 

check_include_files(stdatomic.h FLB_HAVE_STDATOMIC_H)