message: In CMake builds using MinGW, set CMAKE_CXX_EXTENSIONS=ON, i.e.
  use the flag '-std=gnu++11'. Without this, MinGW GCC does not
  expose the functions jn/_jn, yn/_yn which are used by Example
  25/25p.
diff: @@ -90,6 +90,11 @@ include("${CMAKE_CURRENT_SOURCE_DIR}/config/XSDKDefaults.cmake")
  # Enable languages.
  enable_language(CXX)
  +if (MINGW)
  + # MinGW GCC does not expose the functions jn/_jn, yn/_yn (used in Example
  + # 25/25p) unless we use '-std=gnu++11':
  + set(CMAKE_CXX_EXTENSIONS ON)
  +endif()
  if (MFEM_USE_CUDA)
  if (MFEM_USE_HIP)
  message(FATAL_ERROR " *** MFEM_USE_HIP cannot be combined with MFEM_USE_CUDA.")
  