cmake SUNDIALS_C_COMPILER_HAS_ATTRIBUTE_UNUSED examples

AMICI-dev/AMICI ThirdParty/sundials/cmake/SundialsSetupCompilers.cmake :225 

check_c_source_compiles("
  int main(void) {
    __attribute__((unused)) double a = 0.0;
    return 0;
  }
" SUNDIALS_C_COMPILER_HAS_ATTRIBUTE_UNUSED)