message: fixes for CI
diff: #ifndef MFEM_COEFFICIENT
  #define MFEM_COEFFICIENT
  +#include <functional>
  +
  #include "../config/config.hpp"
  #include "../linalg/linalg.hpp"
  #include "intrules.hpp"
  @@ -135,13 +137,13 @@ public:
  /// Define a time-independent coefficient from a pointer to a std function
  FunctionCoefficient(std::function<double(const Vector &)> F)
  {
  - Function = f;
  + Function = F;
  }
  /// Define a time-dependent coefficient from a pointer to a std function
  FunctionCoefficient(std::function<double(const Vector &, double)> TDF)
  {
  - TDFunction = tdf;
  + TDFunction = TDF;
  }
  /// (DEPRECATED) Define a time-independent coefficient from a C-function
  