cmake DEAL_II_HAVE_CXX17_LEGENDRE_FUNCTIONS examples

dealii/dealii cmake/checks/check_01_cxx_features.cmake :589 

CHECK_CXX_SOURCE_COMPILES(
  "
  #include <cmath>
  using std::legendre;
  using std::legendref;
  int main()
  {
  }
  "
  DEAL_II_HAVE_CXX17_LEGENDRE_FUNCTIONS
  )